Pluto’s Kingdom

In the furore following from Pluto’s demotion after Eris’s discovery, a few people argued that Pluto of all places deserved to be called a planet because it had a moon. In fact it has at least five: Charon, Kerberos, Hydra, Nix and Styx, not in that order. It certainly seems to make sense that if a world is hefty enough to have its own companions, it ought to count as a planet, but in fact that isn’t how it works, and there are actually a couple of reasons why having moons almost makes a world less planet-like, if by “planet” you mean a solid or fluid spheroidal body with a relatively strong gravitational pull.

Only two of the universally accepted major planets have no companions: Mercury and Venus. These are notably the two next to the Sun, so the reason may be that they lack the gravitational “oomph” to maintain them. Matter circling either wouldn’t have to be very far out before it felt the Sun’s pull more strongly than the planets’. That said, both of them have respectable gravitas of their own and are far more than just a bunch of rocks loosely bound together. This last is the point really. A small object is less able to hold itself together and is therefore more likely to be a collection of stones or chunks of other matter, highly porous and riddled with caves and liable to lose some of itself or not accumulate nearby bits of matter in the first place. Therefore, in a way, if a body has a few moons, this could be more a sign of it not being a proper planet rather than the other way round. The other reason is basically the same but proceeding from the other end. Many Kuiper Belt and scattered disc objects are binary, and quite possibly more than binary. The same is true to a lesser extent of the asteroids. Being binary is therefore a characteristic of agglomerations of matter which are too small to hold together, but confusingly, having moons is also a characteristic of large planets able to pull loads of stuff towards them which is either already in clumps or forms into planet-like worlds in their own right. Hence Pluto having five moons, one of which is very large indeed compared to the planet (yes, planet) itself, doesn’t count towards its possible planethood.

All this aside, Charon is so large that if it orbited alone it would definitely count as a planet, at least if Pluto does. Earth is notorious for having an unusually large moon, if moon it be, of an eighty-first of its density. Charon far outdoes this, and in doing so consequently outdoes all the other planets in this respect, whose moons are generally well under a thousandth of their mass. Charon’s mass is a little under an eighth of Pluto’s, which is deceptively small as it should be remembered that the diameter relates to the cube root of this figure. After all, Cynthia is a large disc in our sky because it’s a quarter of Earth’s diameter, not 1/81. If the ratio applied to Cynthia and Earth, the former would be considerably larger than Mars, and it might even be habitable, which raises the question of whether such double habitable planets exist out there somewhere. Charon is 1212 kilometres in diameter. Cynthia, like many moons, always shows the same face to us, and the same is true of Charon and Pluto, but in their case the situation is mutual. Both worlds face each other at all times.

I’ve allowed Charon to be overshadowed by Pluto in my own mind, and know relatively little about it. The story of its discovery and naming is quite remarkable. The mythological figure Charon is of course the entity who ferries the souls of the dead across the River Styx into the Underworld, and Pluto being king of the aforementioned domain, one might fancy that the motivation for calling the moon that was clear. However, this is not in fact so. The man who discovered Charon, James W. Christy, actually named it after his wife Charlene Mary, whom he calls Char, and had no idea that the Ferryman was called that too. This gives me pause for thought, because it doesn’t seem to work like one would expect it to naturalistically. It’s reminiscent of the fact that before Saturn was believed to have rings, saturnine herbs were those which had prominent rings, and it’s almost as if the names of celestial bodies are “out there” waiting to be discovered rather than invented, like the non-existent American states of Jefferson and Superior. I won’t dwell too long on this here, but a similar phenomenon is manifested in western astrology where hypothetical planets have been used which have turned out to be real, particularly Pluto.

On 22nd June 1978, Christy noticed that his image of Pluto was not circular, and also that it changed shape on a regular, predictable basis:

Pluto appeared to have a lump on its side which appeared and disappeared. Since the planet is far too big to be irregular, it was correctly concluded correctly that it has a moon, and that that moon takes almost six and a half days to orbit Pluto, or rather, that the two of them take that long to orbit each other. Of all moons and planets in the system, other than small irregular ones, Pluto and Charon are respectively the first and second largest worlds in their companion’s skies, even larger than the Sun in Mercury’s sky (which actually isn’t that large though). Due to captured rotation, that’s also the day length for both Pluto and Charon, and it makes Pluto the only planet to have captured rotation with its satellite, to the extent that it actually counts as a planet, not because of the IAU but because it’s binary and almost orbits Charon rather than the other way round. Axial inclination can also be guessed at fairly reliably with this because the two are likely to circle over each others’ equators, and it’s 57°, exceeding 45° and leading to different variations in day length and the like for the two. Any tilt over 45° involves a peculiar set of circumstances where the polar circles are closer to the equator than the tropics are, though at such a distance from the Sun it’s questionable whether it makes much difference. One thing which definitely does make a difference on Pluto is the atmosphere snowing onto the surface in the autumn and evaporating again in the spring, bearing in mind that the dates for these are more than a dozen decades apart. Speaking of dates, there are 14 205½ Charonian (or Plutonian) days in their year.

The two share many characteristics. Some of these are also shared with Triton, which is closer to Pluto in size and mass than Charon is, but the conditions on the two are even more similar because of their gravitational influence on each other and being the same distance from the Sun, having the same axial tilt and day length and so forth. It’s actually slightly awkward to talk about Charon separately from Pluto, but I’ve written quite a bit about the latter already and don’t want to go over it again. New Horizons managed to take photos of the two together, like this:

This picture is a bit misleading, as it’s effectively taken through a telephoto lens. It wouldn’t be possible to see this similarity near either world because the two are almost 20 000 kilometres apart and Charon is considerably smaller than Pluto even though they are closer in size than any other planet-moon combination. Even so, Charon is notably duller and has a reddish cap over its north polar region, whereas Pluto’s is closer to its equator. This red substance is, however, the same, and seems to have been shed from Pluto and deposited on Charon. Unsurprisingly, it consists of tholins, which are as I’ve said before an organic mixture of dark red tarry stuff which reminds me of the deposits made by herbal tinctures, partly because they actually are quite similar. Tannins in particular spring to mind. To repeat myself from elsewhere on this blog, tholins are the alternative route taken in the Universe by organic chemistry to organic life. The question of how often organic chemistry becomes biochemistry is another question, but there are clearly countless examples of tholins in the Universe judging by how many there are orbiting the Sun. Methane is also deposited on the surface from Pluto. Before any of the stuff gets there, though, it’s been part of Pluto’s atmosphere, and is therefore deposited faster near perihelion. Also, we finally get an answer to why trailing hemispheres are more heavily coated than leading ones: it’s because of gravity. Trailing hemispheres simply bear the brunt of falling material because the material has fallen further by then. The north cap is called Mordor Macula, “macula” meaning “spot”, as in “immaculate” – “spotless”.

Unlike Pluto, whose surface is largely solid nitrogen, Charon’s surface away from the tholin cap is mainly water ice but there are also patches of ammonia hydrates. Also unlike Pluto, there is effectively no atmosphere, so the snowing and sublimating processes on that planet don’t occur here. The south pole is also rather dark, but the north is darker. Although Charon doesn’t have a persistent atmosphere, substances on its surface do sublimate, becoming gas. It’s just that its gravity isn’t strong enough to hold on to any of them. The southern polar region was actually imaged with the help of “plutoshine”, as it was night time there when New Horizons visited, so image processing involved removing the tint of Pluto’s light to restore it to how it would’ve looked if sunlit.

Charon does actually seem to be geologically active, with geysers similar to those on Triton, shedding water ice and ammonia nitrate. This must’ve happened last less than thirty millennia ago, probably a lot less, because the ice deposits are still crystalline and haven’t changed to the glassy form expected after such a long period of time. The different composition of the geyser plumes also means that the moon is different beneath the surface and has geological layers, which was previously controversial as it is quite small. It’s likely that the moon is geologically active due to Pluto raising tides within it, a possibly mutual process, which raises the question of whether there’s substantial heating and an internal water ocean, which it’s becoming apparent is very common in the Universe. Scientists believe that in the distant geological past, it did indeed have an ocean within it but that this froze and expanded, leading to the formation of the enormous canyons visible on its surface in the image at the top of this blog post. This is one way in which water, as a geologically significant compound, behaves differently and leads to different land forms than other substances which melt and freeze. On Earth, water is currently not often a geologically significant “rock”, except at high altitudes and within the polar circles. Beyond the frost line of the Solar System, it often is, and unlike the other liquids, which are often gaseous at Earth-like temperatures, it expands on freezing, leading to geology very unlike ours. Although there are some other substances which expand on freezing, such as bismuth and gallium, they don’t generally occur in bulk. In the case of Charon, water ice is a major and significant mineral which contributes to the landscape and interior in a way something like silicate or carbonate rock does on or in Earth.

More precisely, the reason for those canyons is that as the interior of the moon froze, it expanded and fractured the surface, leading to the formation of a number of features referred to as “chasmata” – “chasms”. These include Tardis, Serenity, Nostromo, Caleuche, Mandjet, Argo and Macross. Many of these have a rather obvious naming scheme, which is fun. Caleuche, which is named after a mythical boat which sails the coast of Chile collecting the souls of the dead, is a Y-shaped canyon thirteen kilometres deep, among the deepest chasmata in the system. Mandjet is thirty kilometres wide, four kilometres deep and 385 kilometres long. Serenity is two hundred kilometres long as a chasma but runs an additional two hundred as an unpaired escarpment. All of these chasmata run around the moon’s equator, separating the northern Oz Terra from the southern Vulcan Planum, which is named after Spock’s planet. Oz is a kilometre higher than Vulcan over its whole surface. Both Oz and Vulcan extend across into the portion of the moon which was dark when New Horizons got there, but it seems likely that each occupies an entire hemisphere. Vulcan is less heavily cratered, suggesting that there’s recently (relatively) been geological activity there which has erased them by remodelling the surface. However, there are some craters and also central mountains, including Kirk and Kubrick. Spock, Sulu and Uhura are also represented thus, as well as Clarke (Arthur C Clarke). The entire area seems to have been covered by a large flow of liquid over the entire hemisphere, probably water.

Other craters include Vader, Pirx, Alice, Organa, Dorothy, Nemo, Skywalker, Ripley, Revati, Sadko, Nasredin, Cora and Kaguya-Hime. I do wonder how people whose religion includes some of these figures feel about the avowèdly fictional characters represented here, but perhaps the day will come when the Vulcan and Jedi world views become official religions too, if they haven’t already. There is another macula, Gallifrey, through whose middle Tardis runs. This means, oddly, that the confusion the Bi-Al Medical Foundation receptionist shows in the ‘Doctor Who’ adventure ‘The Invisible Enemy’ could be explained in a fangirlish way by the presence of this feature, which creates an Ontological Paradox similar to the one created by K-9’s motherboard, introduced in the same episode.

That, then, is Charon, which deserves considerable attention as the largest and best-known of Pluto’s moons. However, there are four more to be covered, and this raises a question: how do they orbit? All other known satellite systems with more than two members consist of a relatively large planet and a number of much smaller moons, and although the orbital dynamics can be somewhat peculiar, such as coörbital moons regularly swapping positions, Pluto-Charon is a different matter. There are two relatively similar masses and other moons in the immediate vicinity. It was calculated at one point that there could be stable orbits in such a situation if an object was at least 3.5 times closer to one mass than the other or if it was at least 3.5 times the maximum separation between the pair, and there are also improbable but stable orbits of various kinds between them such as a figure of eight. Ternary star systems usually have two close companions and a third, much more distant one: this is true, for example, of the Centauri system, where Proxima is much further away than A and B are from each other. The Pluto-Charon system is unique as far as is known in the Solar System in this respect.

Where, then, are the other moons?

This is an image taken by the Hubble Space Telescope three years before New Horizons reached Pluto, and was used to plan the mission. It’s notable that Charon and Pluto actually look fainter in this image than Hydra and Nix, or at least smaller. Styx doesn’t seem to be far away enough to maintain its trajectory. This picture shows that the moons are outside the Pluto-Charon region, separated by a small gap but all relatively close to each other, in an arrangement which reminds me slightly of the TRAPPIST-1 system where several planets are within the habitable zone. They don’t seem to be spaced any way like the Titius-Bode Series and although there is a space between the inner two and the rest, the relative distances of the others are not like those of ternary stars. It also raises two questions in my mind: is this similar to how planetary systems might be arranged around binary stars? Also, is this where Earth’s other moons would be if we had any?

There’s a further surprise. At least two of them are merged double moons themselves, namely Hydra and Kerberos. Going off on a tangent for a moment, bearing in mind that scientists now have sufficient reliable information to establish that two of the small moons of Pluto are former double moons, what the heck do flat Earthers and people who believe, and I quote, “space is fake” think is going on here? Why would NASA, other space agencies and the global astronomical community bother to put in that kind of detail about an entirely bogus cosmos? On the other hand, it is also true that esoteric blind alleys have been known to become highly elaborate, so maybe they think it’s along those lines. Also, fictional universes can be very intricate too. It just strikes me as highly implausible that something like this would be made up and makes me wonder about how flat Earthers think.

Anyway. . .

Hydra and Kerberos are former double moons, and this is evident from their shapes. This is Hydra:

This shape is similar to the comet being studied by the Rosetta probe, and in the comet’s case it’s thought to result from the merging of two bodies. This is that comet, known as 67P:

In the comet’s case, it’s been suggested that the shape results from the heat of the Sun eroding the nucleus. However, each lobe has concentric strata, suggesting that it was originally two bodies which got stuck together. Were it only one, it would have layers indicating a former, more regular form. Hydra is fifty-one kilometres long. Like all the small moons, Hydra is shiny with water ice, and is the outermost moon at a distance of 64 738 kilometres from the barycentre, which is outside Pluto. It’s probably receded from Pluto-Charon due to tidal forces. The name is a bit unusual and sticks out because it isn’t named after a humanoid mythological figure, and this principle also applies to the next moon in.

Which is Kerberos, named after the four-headed (the snake forming the tail has a head) guard dog of the Greek Underworld. Isaac Asimov once suggested that the tenth planet should be called Cerberus so that a mission approaching the Solar System from the great beyond would encounter the system’s guard dog first. To that end, it makes more sense that Hydra be called Kerberos and since the latter was already known to be closer to Pluto than Hydra when it was discovered, its name lacks elegance in a way. There are no good images of the moon:

This image gives the impression that the moon has done something naughty and needs to have its identity protected, but it can again be seen to have two lobes, suggesting again that it’s the result of the collision of two former moons. The two-lobed “dumb bell” appearance is quite common and approached by orbit-swapping moon pairs of moons near other planets. It’s about nineteen kilometres long and averages 57 783 kilometres from the barycentre. This figure combined with Hydra’s gives some indication of how close together the outer moons are, as these are the two outermost and there’s a highly unstable region close to Pluto-Charon, so there isn’t much space between them for moons to exist. Kerberos was named after an online poll and was not the most popular choice, and it’s spelt that way because there’s already an asteroid called Cerberus. The final choice was made by the IAU. Hmmm.

The next moon in, Nix, also has a story behind its name, which has again been re-spelt. Nyx is the Greek goddess of night, but since there was already an asteroid with that name, it became Nix in Pluto’s case, which is the Coptic spelling: “Ⲛⲓⲝ”. There’s actually a pretty good image of Nix from New Horizons:

To me, the brown smudge closest to the camera, which is eighteen kilometres across, looks like tholins, and there are also white bits which I imagine are water ice. Nix is almost exactly fifty kilometres long. Like all the smaller moons, Nix doesn’t have captured rotation but tumbles, so all these four moons have no north or south in the rotational sense.

The innermost small Plutonian moon is Styx, and if you thought Kerberos had a poor image, just look at this:

It can be conjectured to be elongated like Nix and is the dimmest known object in the Solar System at a magnitude of 27. That is, it’s as dim compared to a star like Vega as Vega itself is to the Sun, from Earth of course. I’m a little surprised by this because I would’ve thought Adonis, for example, would be dimmer, since that asteroid is only two hundred metres across, but that’s actually hundreds of times brighter at 18. Styx is a sensible name because crossing its orbit brings one into Pluto’s kingdom, more or less, and it’s also the next moon out from Charon. Styx’s longest dimension is sixteen kilometres, so it’s smaller than the oft-employed Isle Of Wight yardstick. It takes twenty days to orbit the barycentre, 42 656 kilometres away.

All of the outer moons have orbital resonances with each other. Styx is almost in harmony with Pluto-Charon too. This brings up the question of their probable mode of formation. All are grey, unlike Pluto, and are thought to have been formed in a similar manner to Cynthia, with an impact from a large body kicking up débris from the surface which later fell into orbits and coalesced. These orbits would’ve been closer to Pluto than they currently are. Interestingly, three of the moons were named in 1940 in a SF story by Peter Hamilton: Cerberus (sic), Charon and Styx. Their orbits are fairly chaotic and not fixed over millions of years.

Next time I’ll turn to the other largish worlds beyond Neptune. We’re really approaching the end now. Thank you for your patience.

Hail Eris!

It used to be so simple, concordant and ordered. There were nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Hamlet, Neptune and Pluto. Of course, on the whole people didn’t call the one between Saturn and Neptune by that name but my patience with puerile jokes is finite and I actually think making one of them a joke just because it has a ridiculous name does it and science a disservice. My Very Eager Mother Just Served Us Nine Pizzas. Many Volcanoes Erupt Mulberry Jam Sandwiches Under Normal Pressure, which is the one I remember. Those mnemonics are actually quite odd, not just because they’re memorable sentences – it’d be odd for a mnemonic not to be memorable – but because I don’t actually think many people have any problem remembering what order the planets are in. It’s a bit like “Richard Of York Gave Battle In Vain” or “Roy G. Biv”. It isn’t really hard to remember what order the colours of the rainbow are because they blend into each other: orange is reddish yellow, indigo bluish violet and so on. Indigo in fact is just a kludge so they add up to seven. It’s not that it isn’t a real spectral colour so much as that lime green and cyan are too, but don’t get a mention.

I have a dormant project on the Althist Wiki called ‘The Caroline Era‘, where I imagined that instead of history doing a seemingly weird swerve at the end of the 1970s CE, it just carried on going in the same direction, with the post-war consensus being preserved. It turns out to be messy and difficult to contrive circumstances in which this could’ve happened. No fewer than seven major trends would have to have been different beforehand in order for this to have continued, one of which occurred as early as 1820. This alternate history also has different astronomy, not because there’s any difference in the planets, moons and the like but because the attitudes towards them were preserved and the technology available for investigating them advanced more slowly, in a way. Two of the ways in which this manifests itself are in the names of the solar planets and what’s considered a planet.

Back in the day, a planet was considered a large round non-luminous object orbiting the Sun independently, more or less. There wasn’t a firm definition but this is probably what people would agree with if you described them that way. I have already gone over the rather dubious procedures which led to this being changed to something most ordinary people would disagree with. Before this happened, however, astronomers, science fiction writers and others practically had a name picked out ready to apply to the next major planet to be discovered: Persephone. Persephone is kind of supposed to be the name of the planet, except that there’s a long-established asteroid called Persephone too. That said, there are also many duplicate names in the system and it doesn’t seem to have stopped astronomers reusing them. Ganymede springs to mind. Also, there’s a Latin version, Proserpina, which is also an asteroid, discovered quite early. Nonetheless the opinion is expressed that any “proper” planet out there beyond Pluto will not be called Persephone for this reason.

When Eris was discovered, it wasn’t given a name because its discovery was the main cause of controversy over the definition of a planet, which I’ve already said I consider rather silly. Because it wasn’t clear how it should be regarded, and there are different naming conventions for differently-classified objects in the system, it couldn’t be officially named. It was, though, given the unofficial name Xena after a show I’ve never seen called ‘Xena, Warrior Princess’, and its moon was given the name Dysnomia. The problem Eris was seen to pose was that if it were to be admitted into official planetaricity, the chances are that a lot of other similar worlds would also have to be called planets, and we could well have ended up with more than a hundred official planets. Now I have to admit that one of the things which annoyed me about what I now think of as the children’s space horror book ‘Galactic Aliens‘ (my review is on that page) was its portrayal of star systems as containing dozens of planets, which seemed unrealistic to me, but it now appears that it’s merely a question of definition, and the slight sense of disease I feel at this is not widely shared. The IAU decided to redefine “planet” because of Eris, making its name, after the goddess of discord, highly appropriate because that proved to be unpopular with the public. I presume the motive for calling it that was its disruption of the concept of “planet”, and it certainly succeeded in sowing discord when it provoked the turn against Pluto’s planethood among IAU members.

Eris is comparable in size and mass to Pluto and the probable former plutino Triton. Eris is a mere two percent smaller than Pluto in diameter and 27% more massive, which kind of makes the two cross over and means there isn’t much to choose between them. Hence there is a sense of fairness in excluding Pluto as a planet if Eris isn’t alowed to be one either. Nonetheless, if it had been discovered under different circumstances it would almost certainly have been thought of as one. There is no reason why, if you look at Pluto as a planet, as we did for many decades, you shouldn’t also look at Eris as one.

Compare and contrast this with Sedna. Not to diss the world, but it’s only a little larger than Ceres. Its mass is unknown because it seems to have no moon, which is unusual for these objects. It counts as a dwarf planet, to be sure, but Pluto and Eris are on a different scale.

Naturally Eris has never been visited. It’s the seventeenth largest world in the system, and the largest never to have had a spacecraft sent to it or past it. It averages almost 68 AU from the Sun, takes 559 years to orbit and is currently about a hundred AU from us. Sunlight takes thirteen hours to get there right now. At its closest approach, it comes slightly closer than Pluto’s average distance but it doesn’t cross Neptune’s orbit and is therefore not a plutino and doesn’t interact with Neptune. Its maximum distance from the Sun is 97.4 AU, which means it’s currently about as far away as it gets. I suspect that there are a number of Kuiper belt objects whose existence we only know of because they’re currently near perihelion, but this doesn’t apply to Eris. The Sun is currently over nine thousand times dimmer there than it is here. The distance of the world, and in fact I’m going to call a spade a spade and refer to it as a planet, the planet from the Sun is unprecedented in this series. It’s about five dozen times as bright as moonlight at that distance, meaning that finally the idea of a distant planet being so far from the Sun that it’s like night there may finally have begun to be fairly accurate, although a night of a brightness only seen on this planet had there been a fairly nearby supernova in the past few days. Surface temperatures vary between -243 and -217°C, so it doesn’t even get warm enough there to melt nitrogen or oxygen. It’s currently on the low side, and the seasons would be quite substantially determined by its distance from the Sun rather than just its axial tilt, although that’s also considerable at 78° if Dysnomia’s orbit is anything to go by.

Eris is bright. It isn’t like many of the other trans-Neptunian objects (TNOs), which are quite dark and also red. Its surface reflects most of the light back again, which makes it colder than other such worlds at comparable distances, and it’s also unlike Pluto, Charon and Triton in that respect. This is Charon:

. . .which looks quite like Pluto:

(to an extent), which in turn resembles Triton to a certain degree:

All three worlds have tholins on their surfaces to some extent and reflect up to 76% of sunlight. Eris could well be as bright as Enceladus. Something else is going on, or has gone on, there. One thing which very probably does happen is that it has a seasonal atmosphere. The surface is likely to be covered in a layer of frozen nitrogen and methane which will evaporate in a couple of centuries time when spring comes, at which point it will have a tenuous nitrogen-methane atmosphere for the summer, then with the onset of autumn this will freeze and snow onto the surface, once again covering it. This is a five and a half century process though, so we will never witness it. The last time Eris was where it is now was two decades before the Battle of Bosworth Field and three decades before Columbus reached the New World, and each season lasts something like the interval between the first Boer War and the present day, which means it’s just barely within the memory of my grandparents, and I’m middle-aged. That would be the average length. In reality, the winter is the longest season and the summer the shortest, and all seasons are somewhat affected by the considerable axial tilt. My ignorance of calculus makes it impossible to be more precise.

In considering Eris, we’re thrown back substantially onto pre-space age technology. Although there have been many advances in astronomical observation and reasoning since 1957, considering the planet is reminiscent of the kind of observation and reasoning astronomers used to have to use when all they had was what they saw through telescopes. This is not entirely true though, because conclusions were drawn on the basis of the actual space exploration of similar worlds, which didn’t just rely on light and other electromagnetic radiation, and the Hubble Space Telescope made a big difference too. There are also better modelling techniques. Even so, Eris is a dot in a telescope with another dot, Dysnomia, orbiting it, and astronomers have to base most of their studies on those. I’m once again reminded of Chesley Bonestell’s paintings of Saturn seen from different moons where the central subject more or less had to be the planet’s rather than the moons’ appearance because little was known about the characteristics of the moons themselves other than what was implied by their appearance through a less-than-ideal set of telescopes through Earth’s atmosphere, and their movements. Io, for example, was probably never depicted with a volcanic eruption taking place on it until the late ’70s or after. Nonetheless it’s still possible to go a long way with what we’ve got, and there’s even a kind of nostalgia to it. Just as we used to imagine oceans on Venus and canals on Mars, so we can project our wishes onto Eris. For instance, it could have the ruins of ancient alien space bases on it and we’d be none the wiser, although I very much doubt that’s so. Science fiction might be able to colour it in that way, but the genre hasn’t really developed in that direction. The planet is in a bit of a peculiar position because on the one hand it was fêted and imagined in detail for decades before it was discovered – mentioned on classic ‘Doctor Who’ for example – but when it was discovered for real, it ceased to be considered a planet within about a year and the kind of popular culture which existed by then had little space for such a concept as the “tenth” planet. It’s also been stated that not calling it the tenth planet is insulting to Clive Tombaugh’s memory, because he discovered Pluto. Calling it the ninth would be the same, and also wouldn’t make any sense. It’s either the tenth planet or not a planet at all.

The presence of Dysnomia is fairly typical for dwarf planets, which are often binary or at least have moons. Dysnomia is around seven hundred kilometres in diameter and is therefore almost certainly spheroidal. Here’s an image of the two together:

Eris is the brighter light in the middle, Dysomia the left lesser light. Since the moon can be observed to orbit Eris and perhaps also displace it as it does so, the time taken and the distance between the two can be used to calculate the mass of Eris, and the displacement would enable the density and mass of Dysnomia to be found. The moon might be a rubble pile, apparently, which surprises me because it seems too large not to have welded itself together. It was originally unofficially called Gabrielle due to the ‘Xena, Warrior Princess’ thing. Dysnomia orbits Eris once in almost sixteen days, averaging 37 000 kilometres separation in an almost circular path. It’s a lot less reflective, so it may not be made of the same stuff.

It’s possible to say a few of the usual things about Eris which follow from its known size, mass, density and orbit. It has a diameter of 2326 kilometres and a surface gravity 8.4% of Earth’s, which is about half Cynthia’s and close to Pluto’s. Its orbit is inclined 44° to the ecliptic. Its gleaming surface, which is almost uniformly bright, makes it difficult to measure its rotation, but it seems to be fourteen and a half days, making it just a little less than the “month” of Dysnomia. The planet is actually easily spottable through a large telescope. It wasn’t discovered before because its high orbital tilt keeps it away from the ecliptic where other planets generally stay. Even so, right now it is about ten thousand times too dim to be seen with the unaided naked eye, which is about as bright as a Sun-like star would look at the edge of our Galaxy, i.e. about twenty thousand light years away, so it ain’t exactly bright from this distance. It spends about thirty years in each of the maybe four zodiacal constellations it passes through and is currently in Cetus, the Whale.

Eris is not a plutino but a scattered disc object. The scattered disc is not the Kuiper belt, which consists of objects orbiting close to the plane of the Solar System, but comprises objects with highly tilted orbits such as Eris itself and many others, whereas the Kuiper belt planetoids orbit close to the plane of the inner system. The planet, however, still is quite remarkable as it shines forth compared to many of the others in the scattered disc, which have probably yet to be discovered due to their low albedo. It’s a little hard to imagine what could be so exceptional of Eris, it being, like the others, remote from other such objects barring its moon, and other scattered disc objects also have moons, often large compared to their own bulk like Dysnomia. However, discussion of this should wait for another time when I’ll be going into trans-Neptunian objects in more depth.

The surface area is almost seventeen million square kilometres, which is larger than any continent except Eurasia. It has a 26-hour day. It’s higher in rock than many other outer worlds. There’s very little else to say about Eris because so little is known about it, but it’s certainly a fair target for exploration as it’s certainly unusual. The problem is that because the charisma of being declared a planet was denied it, it’s harder to make a case for visiting it. Pluto didn’t suffer this problem because New Horizons was launched a few months before it lost its status. With current spaceflight technology, it would take a spacecraft nearly a quarter of a century to reach it, and once there it would take a radio signal more than half a day to reach Earth at its current distance. It won’t reach its closest approach until the late twenty-third century. The only probe-based exploration undertaken was from New Horizons itself, which was actually further from Eris than Earth was at the time, the advantage being that it was seen from a different angle.

To be honest, it’s a tall order to try to say anything much at all about Eris, as you may have gathered, but there would surely be a lot to say if the opportunity arose to explore it. Right now this seems quite unlikely, and by the time it’s in a position to be visited, we’ll probably be extinct or have lost the ability to launch spacecraft, so don’t hold your breath.

Next time, I’ll be talking about Pluto’s moons, of which there are five known.

Neptune’s Smaller Moons

For Neptune, or rather knowledge thereof, the early 1970s CE were a simpler time. In fact any time between 1949 and 1989 was a simpler time. Back then, Kuiper having discovered Nereid, a smaller and peculiar moon, at the end of the ’40s, Neptune only seemed to have two moons: Triton and Nereid. This state of affairs continued until the end of the ’80s, which was approximately one Neptunian season. Four decades during which the planet only appeared to have two moons. I’ll start with that.

I’ve already mentioned Triton, the oddball moon of the Neptunian system two hundred times as massive as all its other moons put together, orbiting backwards and at an angle, in an almost perfectly circular trajectory. I haven’t mentioned the equally oddball second moon discovered, Nereid, and I say the early ’70s were a simpler time but in fact its own orbit is very peculiar. Nereid has the most eccentric known orbit of any moon. It sometimes feels like discussing the orbit of a celestial body is a bit tangential to the core of its nature, but orbits have important consequences for the nature of planets, moons and their neighbours, and in this case it’s so odd that it would be strange not to mention it, particularly back in 1971 when that was practically all that was known about it. It sometimes feels like the Solar System “frays at the edges” with all this stuff, because things out here are really quite outré compared to the relatively regular innards of this system we call solar. Nereid’s orbit is entirely outside Triton’s, approaching Neptune by 1 353 600 kilometres at its closest and moving out to a maximum of 9 623 700 kilometres distance from the planet. It takes five days less than a year to go all the way round, which is appealingly similar to Earth’s sidereal period. In fact of all Solar System objects its year seems closest to ours. No other moon is remotely as eccentric. At its closest, Neptune would be a little larger than the Sun is in our own sky, and at its furthest, six months later (so to speak), about the size of a lentil on one’s dinner plate. This is probably the result of Triton’s capture, which to me suggests there are other former moons wandering far beyond Pluto or even in interstellar space, or maybe in the “Gap“.

Nereid is small and grey. There is no good image. The best one is this:

Not very impressive, eh?

Unlike Triton, Nereid orbits in the usual direction, as do two other irregular moons Sao and Laomedea, further out. Another moon, Helimede, is a remarkably similar colour but orbits the other way. It’s considered to be a bit that chipped off of Nereid. Nereid itself is about 360 kilometres across on average and may be somewhat spherical but by no means perfectly so. It’s one of several bodies in the system which are right on the border of being round, and is almost as large as the definitely round (sans Herschel) Mimas, but also rather denser. Its shape is therefore hard to determine. Certainly its gravity would be sufficient to pull Mimas-like material into a spheroid, since it’s higher, but that very density may result in the moon being tougher and more able to support its own weight without collapsing. However, its variation in brightness probably means it’s quite irregular in shape and closer to Hyperion in form. Its colour is markèdly unlike that of most centaurs, and it’s therefore probably a “native” Neptunian moon. There’s water ice on its surface.

Proteus is the one which really surprised me. On the whole, the Voyager probes and others only discovered small moons, although Charles Kowal’s discovery of Leda skews that for the Jovian satellites because it’s unusually small for a telescopic discovery of that time. Proteus is actually the second largest Neptunian moon, being somewhat larger than Nereid, and is shown at the top of this post. It orbits the planet at 117 647 kilometres from the barycentre on average in a fairly round orbit, though nowhere near as round as Triton’s. It can be determined not to be perfectly spherical and is in fact not even particularly rounded, with dimensions of 424 x 390 x 396 kilometres. Its surface consists of a number of planes (or plains) with sharp angles between them at their edges and it’s uniform in colour, being somewhat reddish like many other outer system worlds. It was discovered by Voyager, but two months before the space probe got to Neptune.

Unlike Nereid, Proteus was close enough to Voyager 2 to be mapped. As can be seen above, it’s heavily cratered and its surface is therefore likely to be quite old, meaning that nothing much has happened to it in a long time. NASA also had a very steep “learning curve” with Proteus compared to Nereid as it went from being unknown to being mapped within a few weeks, whereas Nereid’s existence has been established for six dozen years now and still there is no map available except possibly the kind of vague albedo feature map which used to be done for Pluto before a spacecraft got there. It can also be seen through the Hubble Space Telescope. It’s fairly dark, probably because its surface consists of hydrocarbons and cyanides. The only named feature on its surface is the relatively large crater Pharos, 260 kilometres across, but due to its somewhat irregular shape this fails to give it the “Death Star” appearance Mimas has. Proteus is also receding from Neptune due to tidal forces and is now eight thousand kilometres further from it than when it first formed. Unsurprisingly, given that it was undiscovered for so long, it’s a lot darker than Nereid.

The inner moons generally are coated in the same material as Proteus. A couple of them are quite notable. For instance, Larissa, which is 194 kilometres in diameter, was accidentally observed passing in front of a star in 1981, leading to the correct but unwarranted conclusion that Neptune has rings. The chances of a moon of that size being seen to cover a star are very small just anyway, but in Neptune’s case it’s even less likely because it moves against the “fixed” stars so slowly, taking almost three months to cover a distance equivalent to the face of the Sun. Larissa’s period is about twelve hours and it orbits only 73 400 kilometres above the centre of Neptune, putting it close to the Roche Limit, where large bodies are torn apart by gravity. It was, however, given a provisional designation in ’81, namely S/1981 N1, so it was accepted as a moon back then. Like the other inner satellites, it’s likely to be a rubble pile, without enough gravity to pull itself together as a solid object. It may be a future ring.

Another somewhat interesting moon is Hippocamp, which is so dim Voyager failed to notice it and had to wait for the Hubble Space Telescope to discover it, which was done by the combination of a number of images as even then it was too faint to be spotted. It seems to reflect less than ten percent of the light falling on it. It’s only seventeen kilometres across.

The closest moon to Neptune, and in fact to any solar gas giant at all, is Naiad, taking only seven hours to travel round the planet. It’s quite elongated at eight by five dozen kilometres, and will either become a ring or fall into the atmosphere in the relatively near future. Thalassa, the next moon out, is coörbital with it. Their orbits are only eighteen hundred kilometres apart but they never approach that closely because they move north and south of each other as they orbit, putting them a minimum of 2 800 kilometres apart. It’s about the planet’s radius from the cloud tops, making Neptune occupy most of its sky. This would make the surface look deep purple if it has a reddish coating like the others.

Like some other moons, the naming scheme has the prograde moons end in A, the retrograde in E and the highly tilted in O. The two outermost moons, Psamathe and Neso, are relatively close to each other, and stand in contrast to Naiad by being the most distant moons of any known planet at forty-six and fifty million kilometres. Neptune’s lower mass also gives them exceedingly long years of around a quarter of a century.

That’s it for Neptune and its moons, and I’ve already done Pluto, so next stop Eris.

The Outermost Planet?

Neptune may be the outermost planet. After the torridity of having to refer to the previous planet by a silly name or bear the brunt of using an unofficial name, it’s nice to have the calm of just being able to call it “Neptune” without the irritation of puerile jokes. That said, things could’ve turned out very differently because one of the names considered for the seventh planet was actually Neptune!

The two planets are the most similar pair in the entire system. That said, having fixated on Hamlet for so long, right now the two don’t look that alike to me. Neptune has no obvious rings, spins more upright and is a much clearer and more vivid (livid?) blue than the hazy and almost featureless Hamlet. The further out a gas giant is, the more likely it is, even if bigger than Jupiter, to look like Neptune. If Tyche exists, it will be blue, and outer planets in other star systems whose stars provide less radiation than about a thousandth of solar intensity at our distance from it are also probably going to look like this, although much dimmer. The above image is actually more colourful than it would look to the unaided human eye, at least at first. At Neptune’s distance, the Sun is nearly a thousand times dimmer than at Earth’s. The logarithmic nature of senses means that this wouldn’t seem as dim as that suggests. It’s still about 360 times brighter than Cynthia ever gets. Moonlight is insufficient to make out colour, but I don’t know about sunlight on Neptune. In a way it’s odd even to consider what colour Neptune would look like to human vision as nobody will ever see it in person and it would appear to be coloured to some species who live on this planet, particularly nocturnal ones.

The Titius-Bode series does not apply to Neptune. It’s actually 30.1 AU from the Sun rather than the predicted 38.8, although Pluto is much closer to that distance. That doesn’t mean Pluto is or isn’t a planet by the way, but that astronomers expected there to be one there and therefore called it one. What’s actually happening there is quite interesting, but I’ll leave that for now. Neptune was discovered in 1846, by which time a large number of asteroids had also been found and Ceres was no longer considered a planet, which led to the idea that Bode’s Law was mere coincidence. The revision which was able to include Hamlet’s major satellites could be seen, again, as a form of pareidolia, where an increasingly vague formula is used to fit observed phenomena which actually doesn’t reflect any real process or effect but just corresponds to the various coincidences. The sequence was originally n+4, with n=0 for Mercury, rather than a simple doubling sequence, and the fact that the asteroid belt intervenes and Neptune doesn’t fit makes the idea that it’s an actual law more doubtful because there are then three out of ten exceptions to the rule. A side issue, probably not important, is the surprising convenience of Earth being at a round ten units from the Sun. The question arises, then, of whether there really is something about Neptune which puts it in the “wrong” place or whether it’s just that the spurious correlation was revealed by it. Most astronomers would agree with the latter possibility.

Neptune is not the coldest planet in the system in spite of being further from the Sun than any other known planet, at least consistently. This is because, unlike the seventh planet, it has a significant internal heat source. It takes 165 years to orbit the Sun, and having a moderate axial tilt this gives the temperate regions four-decade-long seasons. The axial tilt is 28° and the day lasts sixteen hours, which is technically close to Hamlet’s but differs in that the poles don’t spend most of their time pointing towards or away from the Sun. It might therefore be expected to have seasons dominated by the Sun, but this isn’t obvious because unlike its twin, Neptune is heated internally. This leads to Neptune being warmer than the other ice giant at cloud top level. Like the other outer planets, this heat is due to contraction of the planet from the part of the solar nebula it formed from, but in Neptune’s case there may be an extra factor in the form of its large moon Triton’s tidal influence. The centre is at around 7000°C compared to the other giant’s 5000, possibly because Neptune wasn’t disrupted, but it could also be that both planets go through warmer and colder phases and we happen to be living at a time when it’s that way round. I don’t actually know how they arrived at these figures considering that there are theories that the clouds are cold due to insulating convection layers, meaning that heat doesn’t leak out and is therefore presumably undetectable, but this is what they say. Neptune’s centre is therefore hotter than the surface of the Sun.

Regardless of the temperature at the core, the cloud tops are still very cold at around -200°C. Before Voyager 2 got there, it was speculated that the low temperature could give rise to fast winds in the atmosphere because the vibration of gas molecules at higher temperatures was absent, leading to a low-friction environment, and this did in fact turn out to be so. The winds are the fastest recorded in the system at over 2000 kph. At the equator, the average wind speed is around 1100 kph, which is about the same as the speed of sound at sea level on Earth. On Earth, the Coriolis Effect is somewhat significant in generating wind but the main driver is the primary or secondary solar heating and cooling. The Sun heats the air on this planet, causing it to expand, or cooler areas have contracting air over them, allowing the warmer air to move in and occupy the space due to the pressure difference, or in a more complicated process, land and water change temperature at different rates, causing air movement. Although the core of Neptune is far hotter than its exterior, this doesn’t seem to drive the extreme high velocity winds near the cloud tops. My guess is that it’s somewhat similar to a perpetual motion machine, which of course cannot exist. The input from whatever source to the weather systems, such as the Coriolis Effect, tidal forces and the hot interior of the planet, puts the atmosphere in motion and due to the lack of friction that energy is only lost very slowly, and consequently the winds accelerate until they reach the speed of sound, which prevents them from moving any faster. This is not a detailed explanation and may well be completely incorrect. It’s just a guess.

Neptune has more visible banding than the other ice giant, and also has rotating storms in its atmosphere which have been observed to last up to six years. This is far less durable than Jupiter’s storms, but the size and energy input are smaller so this might be expected. Neptune’s Great Dark Spot is visible in the lower part of the picture at the start of this post, but here it is again:

The spot was 13 000 kilometres long by 6 000 wide, and is a hole in the cloud deck. The white clouds around it are cirrus made of frozen methane and were instrumental in enabling the wind speed to be measured. It’s thought that the spots disappear as they approach the equator, which can take years. As I may have mentioned before, the Great Dark Spot was at the same latitude as Jupiter’s Great Red Spot, and this suggests it’s recurrent. If it is, it also shares with the GRS a tendency to appear and disappear. I’ve mentioned elsewhere that it seems to be more than coincidence that planets tend to have a fluid-related feature at this latitude, including Hawaiʻi, Olympus Mons, the Great Red Spot and this storm, which is intermittent, and although I have a vague impression of a pyramid superimposed on the bodies in question with the apex at one pole, I can’t put my finger on why this would happen or whether it actually is more than cherrypicking.

Neptune’s blueness can’t be explained simply through Rayleigh scattering and there must actually be something blue in its atmosphere which isn’t in Hamlet’s, but what this is exactly is another question entirely. Even so, it is true that the methane contributes by absorbing red light. The different hydrocarbon content contributes to it being warmer than Hamlet due to a greenhouse effect, although this is only relative as it’s still at the temperature of liquid nitrogen on Earth.

This is a fairly well-known image of clouds on Neptune above the more generally blue cloud deck. These clouds are frozen methane, but the picture also seems to show that not far below them is a blue haze with a definite level top to it. The clouds are about fifty kilometres above the haze and are casting such definite shadows because the Sun is low in the sky at this point, as evinced by the night on the right hand side of the image. Although the widths of the clouds here varies between around fifty and two hundred kilometres, I don’t know how that scale compares to the clouds in our own sky. It does sound rather larger at first consideration. I’m also tempted to see them as having been streamlined by the powerful winds and feel they don’t have much chance to be wispy, unlike Earth’s cirrus clouds. They’re almost like contrails in a way.

One theory about Neptune’s clouds is that the planet’s atmosphere is effectively a giant cloud chamber. A cloud chamber is a delicately balanced humid atmosphere used to detect subatomic particles, whose energy as they move through it leaves wakes in the form of clouds. This can be created using the steam from dry ice. The planet in question is of course very cold at the height the clouds can be seen, and it’s been theorised that galactic cosmic rays stimulate the atmosphere into producing these streaks. The coolness of the atmosphere makes these things much more significant for Neptune than here, so if this is how it happens, the cause is similar to the high winds. Ultraviolet light from the Sun is also probably responsible for features in the atmosphere, but probably the haze more than the clouds.

The rate of rotation has the same features as that of the bigger gas giants, as the planet does not rotate as a solid body would. The magnetosphere can be taken as a guide to the rotation period if you like, but it isn’t necessarily any more “real” than anything else and we only think it is because we’re from a planet with a solid surface and a shallow atmosphere. The magnetosphere takes sixteen hours, the equator eighteen and the poles twelve. All of this also raises the question of whether it even means anything to assert that Neptune has powerful winds. Maybe that’s just the rotation of the planet, which varies, but it doesn’t mean they actually amount to winds just because different parts rotate at different rates. The understanding of fluid movement used with Jupiter, that they’re cylinders rotating independently, actually cancels out the idea that there are such winds, although there could still be slipstream areas where the wind would be felt.

Unsurprisingly, the interior of the planet closely resembles the other ice giant’s. As I mentioned before, Olaf Stapledon described Neptune, important in ‘Last And First Men’, thus: “. . . the great planet bore a gaseous envelope thousands of miles deep. The solid globe was scarcely more than the yolk of a huge egg.” The upper atmosphere is mainly hydrogen and helium with some methane. Deeper inside is a liquid, becoming solid, layer composed of water, ammonia and methane, and at the centre is a core somewhat larger than Earth made of silicate rock and iron. Like Hamlet, it probably rains diamonds and there are likely to be diamond-bergs floating in the ocean. There may even be a whole layer of diamond deep within the planet.

There being two similar planets of this kind in the system might be seen as coincidence, but in a cosmic context seems not to be. In fact, Neptune-like planets are more common in the Galaxy than Jupiter- or Saturn-sized ones, and the fact that only one spacecraft has ever visited either hampers understanding of a disproportionately large number of worlds. There are nearly 1 800 known Neptune-like planets, notably referred to as “Neptune-like” rather than “Uranus-like”, which makes me wonder again about that ridiculous name although Neptune is more “typical” seeming since it isn’t tipped on its side. Even more common, and absent from the known Solar System, is the intermediate-mass type of planet both smaller than Neptune and larger than Earth. Some of these are much closer to their stars than our own ice giants, and can’t therefore really be classified as such. Nonetheless, this size and mass of planet is common in the Universe.

Getting back to our own Neptune, one surprising finding was that like Hamlet’s magnetic field, Neptune’s is off-centre and at a radically different angle to its axis of rotation. This creates another puzzle because the orientation of Hamlet’s magnetosphere was attributed to its peculiar tilt and misadventure with a large body in the distant past, but given that Neptune’s is also like that suggests that this is irrelevant and makes me wonder if that ever happened, although the tilt does need to be explained. It’s offset by 55% of the planet’s radius and the magnetic poles are 47° from the axis of rotation, yet no explanation based on collisions or close encounters with large objects has been offered so far as I know.

That said, Neptune does in fact show some evidence for this. Discounting Pluto and Charon, the planet has the largest proportionate satellite of any planet in the system but Earth, namely Triton, which is also the only large moon to orbit backwards, and appears to be a captured dwarf planet. Also, the moon Nereid has a comet-like orbit with its closest approach to the planet being much greater than its greatest distance, making it elongated and highly elliptical. Hence one catastrophe may have occurred to Hamlet and another radical event to Neptune, and the question then arises of what was happening in the outer solar system early in its history. Neptunian auroræ are not distributed like terrestrial ones due to the different magnetic field and the presence of rings, which reduces the quantity of charged particles trapped in the magnetosphere. Neptune and Triton also interact magnetically in a similar manner to Jupiter and Io, although not so strongly. There are diffuse auroræ close to the equator to just over half way to the poles, and more definite rings of auroræ closer to the poles, and brighter near the south pole at the time of the Voyager 2 encounter. Neptune has the weakest magnetosphere of any gas giants.

As mentioned above, Neptune has rings. Once Jupiter’s rings had been discovered by the Voyagers, Hamlet already having had them detected, it seemed inevitable that it would have them too, and it has. They were discovered from Earth in 1984 CE but had been seen occulting a star in 1981 in a manner compatible with them not being complete. That is, it was established that there were curved objects orbiting the planet but not that they went all the way round. This is probably because their width varies more than the other three planets’. There was an uncomfortable period in the early ’80s when for me it seemed inevitable that Neptune would be ringed but there was no evidence either way on the issue. I wanted the giant planets to be uniform. For some reason its ringedness is less emphasised than the others’, maybe because it had become routine by that time and it would’ve been more surprising if it hadn’t been.

There is no uniform scheme for naming planetary rings, as can be seen with Hamlet’s. Neptune’s are named after astronomers associated with the planet, specifically Galle, Le Verrier, Lassell, Arago and Adams. Adams is the one with the wider arcs, which are named Liberté, Egalité, Fraternité and Courage. Egalité is split into 1 and 2. Three small moons orbit between the rings, and there’s another ring associated with the moon Despina. In a way it’s quite nice that there’s a French theme to the naming contrasted with the English theme for Hamlet, but I don’t know if it’s deliberate. One really surprising thing about them is that the supposèd “discovery” was actually an occultation by the moon Larissa, so although they were correct about them being rings, they were correct by chance and misinterpretation of an unusual astronomical event. Neptune is a harder target for ring detection than Hamlet, although that is itself not easy, because it moves so slowly against the background with its 165-year orbit. The rings are, like the other ice giant’s, very dark and of course even dimmer due to the greater distance from the Sun. There’s a big contrast in the widths, with the three inner rings being only about a hundred kilometres wide (i.e. their height) and the others being several thousand, which is unlike Hamlet’s much thinner ones. An image of them with the contrast turned up to show details of the structure looks like this:

It’s really come to something when a planet invisible to the naked human eye is made so bright that its glare almost bleaches out the view of its even dimmer rings. This is a ten-minute exposure made by Voyager 2, which was right there, and still the rings are hard to see without that kind of technique. Whether the average human eye could see them is another question, as ours are very good at adjusting to low-light conditions. It still isn’t that low though, at least compared to bright moonlight, but I fear I’m repeating myself. In fact, all the conditions that apply to sunlight on Pluto also apply to it on Neptune because their orbits overlap distance-wise (they don’t literally). Hence the Sun at Neptune’s distance is just a star. The minimum visible object to someone with good vision is one minute of arc across. After that, it’s visible if it’s luminous but not as an actual shape. This is equivalent to a hair’s breadth viewed from twenty-five centimetres away. From Earth, the Sun appears as a disc thirty times that diameter and is therefore very obviously a ball of light. Neptune, however, is thirty times as far away and the Sun could therefore not be seen as anything more than a star, which is effectively a point source of light. This is, however, quite misleading as it’s still many thousand times as bright as any other star in the sky, and might therefore not appear as a point due to its glare. Lighting conditions on Pluto have been likened to those on Earth after a sunny day shortly after sunset, so the same kind of thing can be expected on Neptune and its moons. In other words, you’d probably hardly notice it at all after a while and it would look like broad daylight, except that the actual illumination is only a thousandth that of the Sun’s here. Looking at it from the other end of the telescope, as it were, Neptune is the only planet in the system, taking Pluto as a non-planet, which is never bright enough to be seen. Its maximum brightness is something like four times dimmer than it would need to be to become visible. Of course there will, as with other celestial bodies, be other species who can see it and in fact Galileo saw it, through a telescope of course, but didn’t notice it was a planet. Likewise, it was reported that it had rings shortly after it was discovered but in this case it was probably an illusion.

Neptune has a rather odd array of satellites. At one point it was thought that Triton might be the largest in the Solar System, and as I mentioned above it orbits backwards compared to most other moons. Nereid has a very eccentric orbit. Up until the 1980s, these were the only two moons known, but Voyager 2 surprisingly discovered a moon, now called Proteus, which is actually larger than Nereid, making it the largest object discovered by the Voyager probes. Due to the mistake leading to the accidentally correct conclusion that the planet has rings, the moon Larissa was also detected in 1981 but it wasn’t realised that this had happened, rather like Galileo and Neptune itself. Voyager 2 found another five, including Proteus, and a further six were discovered this century. Neptune also holds the record for having the most distant moon and the longest time taken for that moon to orbit, Psamathe, which is fifty million kilometres from it and has a period of almost twenty-five years. There are various interesting things going on with Neptune’s moons but that can wait until my next post.

Probably the most prominent appearance of Neptune in science fiction is in Olaf Stapledon’s ‘Last And First Men’. Published in 1930, the science is well out of date, although the description of a yolk in an enormous egg is valid. In this account, our distant descendants are living on Venus an æon hence when they observe a mass of gas on a collision course with the Sun which will cause the Solar System to be disruptd and the Sun to become what we would probably call a red giant the size of the orbit of Mercury. Humanity decides, though not en masse, to escape to Neptune, where it has to contend with enormous gravity and pressure, and first a very cold climate followed by a very hot one. Humans cease to be intelligent and take four hundred million years to evolve into a sentient form again. This is partly because their lifespan is much longer, as most species live at least one Neptunian year. They ultimately become superhuman beings who notably have ninety-six genders and a life expectancy of a quarter of a million years. I find this section of the novel, if that’s an accurate description, to be a particularly satisfying example of speculative evolution, although one which has been left standing by scientific discoveries about the planets involved.

That’s probably a fairly adequate introduction to Neptune. Next time: Triton.

Planet Hamlet

Look here for an explanation of the post title. At least for this post I shall be calling this planet Hamlet rather than the silly name. So far as I know, nobody has ever called it that before and it may not function well as a viable official name, although I think it would. Although there may be issues of cultural imperialism, the character as portrayed in the play in question is in a sense global property. On a different note, it has an even lower population than a hamlet.

Hamlet used to fascinate me inordinately as a child, probably for two reasons. One is that it’s blue. In fact, Neptune is if anything bluer, the image above being false colour, but James Muirden the astronomer commented in his book that he definitely saw it as having a blue tinge even though everyone else seemed to see it as green. The border between green and blue seems to be more disputed than most colour differences, and it’s worth remembering that colour terms in other languages often vary, and also tend to occur in a particular order. I presume that Japanese calls the colour in question “青”, as does Mandarin (kind of). The other reason is that for whatever reason, Hamlet is the most obscure planet, being mainly used as the butt of jokes because of its name, which makes it intriguing and a target for the imagination. Hamlet is also only a little denser than water, and at the time of the 1930s (CE) encyclopædia I was getting my info from, its density seems to have been estimated as the same as water, suggesting to astronomers at the time that the planet was a globe of liquid. In 1977, I wrote a story called ‘A Holiday On Uranus’ about exactly that, set in 2177. I remember it fairly vaguely, but in it Hamlet was inhabited by intelligent fish-like beings living in its vast ocean and there was a security scanner used at the spaceport which used terahertz radiation to reveal the surface of the body in clothed people, which was eventually invented for real. Travel to the planet was at near the speed of light. I also imagined slavery in the Saturnian system and cruel and oppressive measures being taken to modify the bodies of Saturnians to make it impossible for them to rebel in an analogy to the Atlantic slave trade. I still have it somewhere I think.

At that time it was still possible to project one’s imagination onto the outer Solar System in such a way, although my view was clearly influenced by the fact that most of what I’d read about Hamlet had been written in the ’30s. Also, in one of those odd random associations one gets as a child, Bing Crosby’s ‘Little Sir Echo’, about a personified echo who was “ever so far away”, always used to make me think of someone living there, and I even went so far as to calculate how long it would take sound to travel the distance from Earth to the planet and back, which is around five and a half centuries. I also imagined a steam locomotive travelling there, which would probably take about a millennium, though that’s a guess. It strikes me that all my imaginings about Hamlet were extremely outdated even for the time I was making them.

Back in Stapledon’s day, and he was chiefly active in the 1930s as far as popular fiction was concerned, the giant planets weren’t considered to be gas giants, but extremely large rocky planets with thick and deep atmospheres. Consequently he was able to imagine Neptune in particular, and also to a limited degree Hamlet, as planets inhabited both by native life and the descendants of life from Earth, and given the increased radiation from the Sun æons in our future, Hamlet has agriculture at its poles, the equator being too hot, suggesting that at that point its peculiar rotation had yet to be discovered.

This brings me to the first real point about the planet: it “rolls around” on its side. Hamlet does not rotate “upright” like most other planets. It doesn’t even rotate at a somewhat tilted angle. Instead, each pole spends a season of the seven dozen-year long orbit pointing towards and at another time away from the Sun, as its axial tilt is 98°. This means that for most of the surface, with the exception of the equatorial region, there are forty-two years of daylight followed by another forty-two years of night. Hamlet does, however, rotate properly every seventeen hours, so at the equator it would have a normalish day with sunrise and sunset. This zone is about fourteen thousand kilometres wide. If it was much closer to the Sun, this peculiar arrangement would lead to very extreme seasons, but Hamlet is actually colder than the next planet out, Neptune, at -224°C. It has the coldest average temperature of any of the planets in the system. This anomalous situation is thought to be caused by the same incident which tilted it so extremely. It’s believed that a major impact or close encounter between a massive object and Hamlet knocked it onto its side and stirred up its atmosphere to the extent that the warmer layers nearer the centre of the planet, where the temperature is about 5000°C, ended up circulating towards the cloud tops and radiating the heat which in other gas giants is insulated from space by thousands of kilometres of not very conductive fluid. It might be thought that the reason is that half the planet is in darkness for forty-two years at a time, but this is not in fact the reason. Hamlet is so far out that it doesn’t really make as much difference to the temperature, and like many outer worlds the internal heat is a major contributor to the climate and weather. However, Hamlet is smaller than the two inner gas giants and has no significant tidal forces to generate heat, so it would in any case have a much cooler interior even without the incident which stirred it up.

When he discovered the planet, William Herschel thought it was probably a comet. It’s remarkable in being the first planet to be consciously discovered in historical times. There is a sense in which Venus was discovered when it was realised that the Morning and Evening Star were identical in the thirteenth century, which also led to it being given that name because the Morning Star was dedicated to the goddess, but an entirely new planet had never been discovered before. Remarkably, Herschel lived to the age of eighty-four, which is the same length as Hamlet’s year. Asteroids began to be discovered about twenty years later. The planet often seems to be passed over. For instance, there are relatively few works of SF which feature it. One exception is Fritz Leiber’s ‘Snowbank Orbit’, a 1962 short story in which the spaceship Prospero ejected from the inner system by an explosion in a battle attempts a slingshot orbit around Hamlet to bring it back inward. This was before such a manœuvre had been attempted for real as far as I know, but is now common, though not round the planet in question. Leiber tends to focus on Shakespeare, so his inclusion of Hamlet in that tale is probably due to its own naming theme. I haven’t read it all, but suspect that the planet only really participates in the plot as a distant “roundabout” rather than a planet in its own right. To be fair, so little was known about the place back then that it might not have had much opportunity to be anything else, although it’s all about imagination and Leiber was substantially a sword and sorcery author as much as an SF one. Cecelia Holland’s ‘Floating Worlds’ novel does have it as a proper location though. I actually owned that book for decades but never got around to reading it before I ended up giving it away, so I can’t enlighten you on its content.

The key concept here, then, seems to be that Hamlet tends to be ignored to a much greater extent than other planets, except for the obvious occasional puerile comment. Is this fair? Is it just that the silly name puts people off taking it seriously, or is there something about it, or perhaps all the other planets, which lends itself to being ignored? Is it the Basingstoke of the Solar System? Come to think of it, is Basingstoke really that boring? Am I being unfair? All that said, Hamlet as a planet, as opposed to our relationship with it, is indeed unusual because of the fact that it orbits on its side, if for no other reason. It’s also the first planet to be found with rings after Saturn, within my lifetime in fact, and its rings are notably different to Saturn’s, being darker, thinner and more widely spaced. Its moons are, uniquely in the Solar System, not marked by any outstanding features. Neptune has the kudos of being the outermost planet if Pluto isn’t counted as one, and for twenty years at a time Neptune really is the outermost due to Pluto’s peculiar orbit. Neptune also has unusual moons and the fastest winds in the system, but I’ll deal with all that when I come to it.

It is, however, worth comparing the two worlds, as they’re probably the two most similar planets in the Solar System. I’ve kind of been here before. Both are roughly the same size, very cold, the same density and have similar day lengths. They also have similar colours and compositions, and their size and density dictate that their cloud top gravity is similar. Although Hamlet is the colder, the difference is only about ten degrees, bearing in mind, however, that ten degrees is a bigger difference at such a low temperature than it is at room temperature and more like a difference of thirty degrees for us.

Here’s the picture I posted last time:

This is Hamlet as it looked to Voyager when it got there in ’86. The equinox occurred in 2007 so this is something like twenty years off from that, a quarter of a “year” or so away from that point. It’s exceedingly featureless and fuzzy looking, unlike the much clearer and more vivid Neptune:

It’s possible that the haze in the atmosphere of the closer planet is seasonal, but this rather uninspiring view is enough to make one understand why it tends to be ignored. After all, just imagine if a space probe costing millions had been dispatched all the way to the place and it had come up with nothing but for the greenish cueball image shown above. Fortunately, Voyager visited all four gas giants and is to date the only spacecraft ever to have visited either Hamlet or Neptune. It took four and a half years to travel the distance from Saturn to Hamlet and at the time it got there, January 1986, the planet was invisible to the naked eye. Hamlet dips in and out of visibility because of its distance and orientation, but is bright enough to be visible as a faint “star” some of the time to people with good eyesight who know where to look. In order to get a good look at Titan, Voyager 1 had manœuvred itself out of the plane of the Solar System and visited no planets after Saturn in late 1980, but Voyager 2 went on to cover Hamlet and Neptune. This means, of course, that the planet didn’t get as much attention as the previous two in any case. There were also imaging challenges. The rings are as dark as coal and the moons are not only dark but also dimly-lit compared to Jupiter’s and Saturn’s. Moreover, the velocity with which Voyager 2 moved through the system marred many of the images with motion blur. This brings up an important issue often raised by conspiracy theorists about NASA. Images taken by space probes are, as far as I know, always processed from the raw form in which they’re received, for this kind of reason. There may be too much or too little contrast, and in this case the problem was that the blur had to be filtered out. I have little idea regarding how this was done, as I would’ve thought that blurring would mean that some features would have obliterated others completely owing to brightness, but maybe not. I do know it seems impossible to get rid of a different kind of blur with processing in that way, namely when things are out of focus, because otherwise an out of focus image could be drawn which would appear to be in focus to someone with myopia, and that doesn’t happen, I’m guessing because of entropy. However, motion blur is not the same thing. Techniques of processing the blur have also improved since 1986, so it’s been possible to extract new information from the data received at the time. In the case of Hamlet I’m tempted to say that it hardly matters because so little detail is apparent, due not to motion blur but the basic appearance of the planet itself.

Another aspect of Hamlet’s appearance is that for human eyes the green-blue colour tends to dominate and make details hard to see. This is similar to the way a clear daytime sky on Earth, so to speak, looks bluer than it really is to many people. This sounds like nonsense, but I have to interject a personal note here that I don’t actually see the sky just as blue, and this is an issue which has come up repeatedly and I haven’t been able to resolve satisfactorily. When I look at a cloudless blue sky in broad daylight, I see large purple “splotches” all over it. These are not directly linked to my vision because they stay in the same parts of the sky when I look around, so it isn’t a question of glare creating an optical illusion due to the blood in my retinæ. It may be connected that in fact the Rayleigh scattering responsible for the bluish colour of the sky isn’t confined to blue wavelengths but actually affects indigo and violet light even more, and I suspect that what I’m seeing is uneven scattering of these higher frequencies. I don’t know why I would notice this more than other people. I wouldn’t go so far as to say that I see the sky as purple or indigo, but it definitely doesn’t look merely blue to me, and for some reason nobody else has ever mentioned this, so I presume they don’t or can’t see it. Nonetheless, if the human eye were equally sensitive to all wavelengths of visible light, the sunlit sky wouldn’t look blue to anyone but more indigo.

I’ve never seen Hamlet with a telescope or anything else, but only via images processed imperfectly for human colour vision. Through violet, orange and red filters, the globe is banded in the same way as Jupiter and Saturn are, though more subtly. The green and blue colour of the atmosphere, however, drowns this out to the unaided human eye. I’ve previously mentioned conspiracy theorists in connection with the question of NASA image processing. Flat Earthers would have the same problem explaining models of Hamlet’s atmosphere as Titan’s, because of the dominance of the Coriolis Effect. Hamlet is very cold indeed, unlike Jupiter and Saturn has only a weak internal heat source, and unlike all other planets in this system orbits on its side. This means that models of its atmosphere correctly show the movements of clouds in a counterclockwise direction dominated by the Coriolis Effect. Note also that these models do not depend on the actual existence of the planet itself, since they’re merely an extrapolation of what happens in a fluid body of Hamlet’s character. The movements are dominated by the movements of the planet itself and not by heat from inside or outside, in spite of the fact that entire hemispheres are daylit for forty-two years at once while their antipodes are nocturnal for the same period, and it might be thought there would be a big temperature difference driving the winds, but there isn’t. This is difficult for flat Earthers to explain because of the rotation of weather systems in our own atmosphere being clockwise on one side of the Equator and counterclockwise on the other.

Hamlet has a number of unusual features which are difficult to explain simply. It rotates on its side, the magnetic field is neither oriented towards the poles or particularly away from them and originates from a location about a third out from the planet’s centre. It’s also colder than expected, and the moons are unusual as well. The most popular explanation is that a roughly Earth-sized body collided with the planet and still has much of its material within it, knocking Hamlet off its axis, changing its composition and causing the formation of carbon monoxide from some of the methane, in other words burning the atmosphere via incomplete combustion due to low oxygen level. Although this is also used to explain the strange magnetic field, I don’t know the connection. Maybe no-one does. This peculiarity also means that unlike any other known planet, Hamlet’s auroræ are equatorial rather than polar, although they do occur around two localised areas on opposite sides of the equator.

One thing I seem to have been right about is that Hamlet contains a vast water ocean, although it is mixed with ammonia, altering its freezing point. Of Neptune, a rather similar planet in many ways, Olaf Stapledon once said, “. . . the great planet bore a gaseous envelope thousands of miles deep. The solid globe was scarcely more than the yolk of a huge egg”. Hamlet and Neptune are by far the two most similar planets in the System, and this is equally true of both. A major fact about both which is almost completely ignored is that it rains diamonds. What happens is that methane is compressed, squeezing out the hydrogen and causing the carbon left behind to form into diamonds under the extreme pressures. These then fall through ever-hotter layers towards the core, where they vapourise, bubble up through the ocean and recrystallise at the top. This also means there may be “diamond-bergs” floating on the ocean. I used the tendency for gas giants to form diamonds in this way in my novel ‘Replicas’, where diamonds have become a monetarily worthless byproduct of the deuterium and helium-3 mining industry on those planets. ROT13’d text spoiler: Zryvffn raqf hc bjavat n qvnzbaq znqr sebz ure cneragf’ erznvaf, fuvccrq onpx ng terng rkcrafr sebz Nycun Pragnhev gb Rnegu, juvpu vf cevpryrff gb ure ohg nf n cenpgvpny bowrpg vf cenpgvpnyyl jbeguyrff. https://rot13.com/. The diamonds may also be floating in a sea of liquid carbon. If this is so, or if there’s a whole geological layer of diamond, it could explain why the magnetic field is so different.

It takes over two and a half hours for a radio signal to pass between Hamlet and Earth, and the round trip is of course twice as long. Voyager 2’s transmitter is about as powerful as the light bulb in a fridge at 23 watts. This is stronger than a mobile ‘phone signal but way weaker than most radio stations. It works over such a long distance because the dishes used are aimed directly at each other, the frequency is free of interference by other human-made signals and the antennæ are very large. This could’ve been mentioned at any point in a number of my recent posts, but it may as well be here. In the case of Hamlet, this single spacecraft is responsible for practically everything the human race knows about the planet, and it relies on that tiny gossamer thread of a radio signal sent in the mid-’80s from two light hours away by a transmitter as weak as a dim filament light bulb. The initial baud rate was about 21 kilobaud, reduced in the end to a mere one hundred and sixty bits per second. They’re pretty amazing ships.

The Voyager mission to Hamlet was overshadowed by tragedy. Its closest approach took place on 24th January 1986, when I was at the height of my arguments with the fundamentalist Christians I met at university (that is relevant, as you’ll see). The Challenger disaster occurred on 28th, and was reported some time in the afternoon. I first heard of it as I was queuing for dinner at my hall of residence, and the kind of “head honcho” Christian student responded that it was “good” because it would persuade people to focus on and spend money on more pressing things. Whereas that’s a common and valid opinion I happen not to share, there’s a time and a place, and I get the impression he was saying that for shock value, which doesn’t seem very Christian by any internal standard. That, then, is my abiding memory of the Challenger disaster, and regardless of the value or priorities of NASA’s Space Shuttle program, the fact remains that seven people lost their lives that day, and of course anyone’s death diminishes us all.

A tangential result of Challenger was that it eclipsed the news from Voyager 2. It was also intimately connected with it in that NASA was inundated with letters requesting that the newly discovered moons be named in memory of the Challenger astronauts. This didn’t happen, even through coincidental Shakespearian characters having the same names. It was a factor in this naming proposal that there was a teacher on board, as many people who were children at the time were watching the launch live on TV due to this connection. It’s also a little-known fact that NASA almost sent Big Bird of Sesame Street, in character, on this flight. In 1988, the IAU, an organisation I currently like less and less the more I hear about it but maybe I’m being unfair, and it is after all an organisation and those are usually bad in some way, voted not to adopt the names of the astronauts for moons because they weren’t international enough. This might seem to make some sense until you consider that they’re actually named after Shakespearean (sp?) characters, which are of course associated with England, so their decision didn’t actually make much sense. However, at least some craters on the far side of Cynthia got named after them.

Hamlet has rings. Although they seem quite different to Saturn’s from a distance, close up pictures are hard even for experts to distinguish between at first glance once the image’s dynamic range has been boosted, because they show the same ringlet structure and there are also at least two shepherd moons, Ophelia and Cordelia. The rings are labelled using Greek letters and numbers, apparently without particular regard to their order. From inner to outer they’re referred to as ζ, 6, 5, 4, α, β, γ, δ, λ, ε, μ and ν. I presume this anomalous order is connected to their order of discovery because the way I remember them from the early ’80s they were named from α to ε. This also seems to continue the tendency to call things to do with the planet odd names, as it seems more logical either to number them or give them letters but not mix the two. The outermost two are red and blue respectively and the rest are dark. The first five, α to ε, were discovered on 10th March 1977 when the planet crossed in front of the star SAO 158687 and it blinked on and off regularly on either side of the planetary disc. However, a ring had been reported much earlier, by William Herschel, although this may have been imaginary because they’re very dark. The ν (Nu, not “Vee”) ring is between the moons Rosalind and Portia, so they also count as shepherds. The fact that most of the rings remain very narrow but don’t have shepherds is unexplained. Before their discovery, only Saturn was thought to have rings. After Jupiter was also discovered to have a ring in 1979, the question was whether Neptune would be the odd one out in lacking them. From that point onward, I assumed Neptune had them. Nobody knows what they’re made of, except that they can’t be ice, because their colours are unusual and don’t yield definite spectra to go on. Their darkness suggests they’re carbon-rich, and in conjunction with the probable diamond-bergs and liquid carbon ocean show that Hamlet is well on its way to being a carbon planet.

Most of the light is reflected by the ε Ring, which is also the most elliptical and the one closest to the equatorial plane. It’s brighter in some areas than others due to that eccentricity and varies in width. It’s possible that this variation translates into arcs – curves – rather than rings for other planets, perhaps orbiting other stars, or maybe Neptune. I can assure you that by the time I come to Neptune I will know if this is so. This is the ring with the first discovered pair of shepherds. The next brightest rings are α and β, which also vary in width, being widest 30° from their furthest points from Hamlet and narrowest 30° from their nearest. It’s probably coincidence that these angles correspond to those of the planetary magnetic field, or if not, something to do with a similar but separate dynamic process. Both these rings are somewhat tilted and are ten kilometres wide in some places, which raises the issue that they were detectable from three milliard kilometres away even though they were smaller than the Isle of Wight. The γ Ring (I’m just going to deal with these in alphabetical order, which means mentioning the 1977 ones first) is narrow, almost opaque and thin enough to make no difference to stars crossing when it’s edge on. This also means it isn’t dusty. The inner edge particles orbit six times for five of Ophelia’s orbits, so there seems to be a relationship there. As for δ, it’s circular, slightly tilted and may contain a moonlet because it seems to have waves in it. It has a more opaque and narrower outer part and a wider and more transparent inner side, which seems to be dustier.

Before Voyager 2 got there, the team who discovered these first five rings found a further three rings by the same method. For some reason these are known as 4, 5 and 6 even though five were already known by that point and there was a Greek letter naming scheme going on from the same team. I don’t understand this, but there it is. Voyager 2 found another two, fainter, rings, the naming scheme going back to Greek letters, and in this century the Hubble Space Telescope found two more. Rings 4, 5 and 6 are up to dozens of kilometres away from the equatorial plane and are inner and fainter to the ones discovered in ’77. They’re also narrower and don’t occult starlight edge-on. The μ Ring is blue and contains the moon Mab, around which it’s also brightest so the chances are it’s made of bits of that moon. These rings are dusty. Finally there’s 1986U2R, because of course it would be called that wouldn’t it?

The rings don’t form a stable system and given what’s known about them should disperse within a million years. However the fact that all the other gas giants have rings suggests either that having rings is normal for such planets or that they’re temporary but very common. Hamlet’s system generally, including the moons, is not so dominated by ring-related factors as Saturn’s although there are several harmonies, operating between small inner moons and the rings rather than the larger classic moons observable from Earth. A moon the size of Puck would be enough to provide the material for the rings, and Mab is actually currently breaking up and forming another ring, so it isn’t that peculiar. There are probably moonlets up to ten kilometres across within each of the rings. I presume the dimness of the sunlight out there combined with the darkness of the satellites and other material makes them harder to detect optically than small moons of Jupiter and Saturn.

Getting back to Hamlet itself, it’s methane which gives it that colour, but the atmosphere is in fact mainly hydrogen and helium like the other gas giants. It’s the second least dense planet and has a cloud top gravitational pull of only 89% of our sea level gravity. There are four layers of cloud corresponding to increasing temperature and atmospheric pressure. At slightly above sea level pressure, there are methane clouds. Considerably further down are the deepest clouds which have been actually observed, where the pressure is equivalent to the Earth’s ocean’s sunlit layers’, and are made of hydrogen sulphide. Appropriately for the planet’s official name, these would stink of rotten eggs. These share the layer with clouds of ammonia, which has an acrid, stinging odour. Below that is ammonium hydrosulphide, and finally, at a level where the pressure is equivalent to about four dozen times our sea level pressure, there are clouds of water vapour. The atmosphere is probably the most featureless of any solar planet’s, but does show the occasional white cloud, as can be seen in the photo at the top of this post. It’s also quite clear compared to all the other gas giants’, Titan’s and Venus’s, though not ours or the Martian one. I would expect there to be a level where one would find oneself completely surrounded by blue-green with various species of cloud. There are also traces of complex hydrocarbons as would be found in mineral oil and natural gas on Earth. Unlike other collisional atmospheres, Hamlet lacks a mesosphere, which is normally found between the stratosphere and thermosphere. There is a hydrocarbon haze in the stratosphere.

The chief distinguishing feature of Hamlet’s atmosphere is its featurelessness. Voyager 2 only detected ten clouds over the entire planet as it flew past. All the other gas giants have more stuff going on in theirs, and this is probably why it took so long to work out its rotational period of seventeen hours. There is a whiter polar cap from around half way between the equator and the poles, which swaps over between north and south as the orbit wears on. Voyager 2 was unable to observe the northern hemisphere because it was night there when it passed, so not only has Hamlet only been visited once but also half of it hasn’t been observed close up at all. In the decade or so after Voyager left, things started happening in its atmosphere but of course they couldn’t be seen as well as they would’ve if they’d taken place when it was there. I feel like there’s a kind of theme emerging here. Also, astronomy has only been advanced enough to make much meaningful sense of what’s going on since the 1950s, which is less than an entire orbit ago, so a whole cycle of seasons has yet to be observed. There has been a dark spot like the one on Neptune, and there are thunderstorms. It’s also possible that there’s a convection layer blocking the internal heat from the outer reaches of the planet.

So that’s Hamlet, such as it is. Next time I’ll be talking about its moons. I have two questions for you though. Did you feel that avoiding the name “Uranus” made you feel differently about this planet? I’m not sure about calling it “Hamlet” either, but that does at least circumvent the issue. Could you think of a better name or is it a bad idea to fixate on it so much?

The Jovian System

I started this series of posts with a survey of our Solar System itself and I’m going to do the same with Jupiter and its moons. When Steve suggested this project, he also suggested working outward from the Sun. The problems with doing this become very evident once one gets to Jupiter, although they were already there with the asteroid belt.

Just with the asteroid belt, I mentioned that although the average distances from the Sun can be organised into bands according to their ratio to Jupiter’s “year” (the official name is “sidereal period”), this isn’t evident at any one moment because many of their orbits are markèdly elliptical and an asteroid in, say, the Hilda group near the outer edge of the belt may well approach the Sun at its closest at the average distance of a Flora asteroid near the inner. Vesta and Ceres seem to approach each other to within four million kilometres, and this will sometimes happen, but lines drawn between each closest approach (perihelion) and the Sun are different lines and the tilt of their orbits also differs, so it isn’t like the system is a flat surface with all the orbits in a plane with their ellipses lined up precisely, or even approximately.

When it comes to Jupiter, a separate problem begins to become evident. All four of the gas giants have extensive satellite systems, and these moons orbit at various distances from the planets, and therefore from the Sun. A moon which is closest to the Sun at one time will be the furthest from it at another, and some of them even regularly swap orbits. It’s actually worth considering this in detail because of what it illustrates about the nature of the systems in general. It’s not much of an exaggeration to say that each of the four planets and their moons is like a mini-solar system in its own right. Perhaps unexpectedly, the system with the most moons is Saturn’s, not Jupiter’s, even though Jupiter is larger, more massive and closer to the asteroid belt. However, for today I’ll mainly be considering the Jovian system rather than the others.

Just before I get going on that, there are “rogue planets”, which in a sense are technically not planets at all, wandering through interstellar space independently of specific stars. These may well have their own satellite systems, and are in a sense “failed stars” because they’re too small to shine, but may even so be several times Jupiter’s mass. Jupiter is therefore in a sense almost our second local “solar” system. Incidentally, there seems to be a gap between the largest planets and the smallest stars, in that the former are much less massive than the other, and there’s also a gap in the sizes of the two types of body because planets tend not to get much bigger than Jupiter in diameter. Above that point, the gravity increases and compresses the substance of the planet more, although there are also examples of planets so close to their suns and therefore hot that they become “puffy planets” which are far larger but also much less dense than Jupiter.

I’ll start with Sinope. Sinope is the most distant moon of Jupiter, and has a surprisingly long astronomical history. It was discovered in 1914 and although it’s quite small, no moon has since been discovered which orbits further out, in spite of today’s space telescopes, the several space missions sent to and through the Jovian system and the discovery of other moons which have turned out to be much smaller, so it was quite an achievement to do that over a century ago. Sinope orbits an average of 24 371 650 kilometres from Jupiter, which is a figure more precisely known today than before. Its eccentricity is, however, considerable, at 0.3366550, meaning that its maximum distance from Jupiter is around 32 576 000 kilometres, which is only a sixth greater than the gap between the orbits of Venus and Mercury at its own aphelion (greatest distance from the Sun). The diameter of that orbit is therefore almost 49 million kilometres, which is comparable to the distances between the orbits of all the inner planets.

Sinope is important because it can be thought of as marking some kind of outer limit to the Jovian system. If we could see that orbit in the night sky it would look larger than the Sun to us. Since it’s further away from us, this means the Jovian system is also literally much larger than the Sun. Sinope takes over two years to orbit Jupiter. There is a large asteroid in the belt named Hilda, whose diameter is 170 kilometres and has an aphelion of 678 million kilometres. Sinope, assuming it to be orbiting in the same plane, takes on average 24 371 650 kilometres off Jupiter’s distance from the Sun, meaning it will be somewhere around 38 million kilometres from Hilda, perhaps less (or perhaps more). Hence Jupiter’s outer moons are actually not that far from the outer asteroid belt. On the other side, Sinope adds the same distance to Jupiter’s orbit and Saturn’s outermost known moon can be taken into consideration, taking it out to 841 million kilometres from the Sun, and Saturn’s apparent counterpart, the as-yet unnamed S2004 S26, approaches the Sun to within 1326 million kilometres, leaving a gap of just under 485 million kilometres. The gap between the two systems is quite small.

Incidentally, another moon, Pasiphaë, is slightly further in but also more eccentric than Sinope, so it can sometimes get even further out.

The magnetosphere also needs to be taken into consideration. Jupiter has a strong magnetic field which starts to interact with the Sun far in front of the position of the planet itself, and also trails behind it in a tail longer than the sunward side. This amounts to eighty radii of the planet to the bow shock, which is the surface where the speed of the solar wind suddenly drops in response to Jupiter’s magnetic field, and is named after the wave in front of the bow of a ship. The bow shock also extends “above” and “below” Jupiter’s orbit by about the same distance, making it the biggest “bump” in the system. The shock is located about six million miles inward of the planet, which is within the satellite system. However, the magnetotail is another matter. The bow shock is actually compressed by the solar wind, so the magnetotail is much, much larger. The entire magnetosphere is somewhat similar to a teardrop shape viewed in cross section perpendicular to the orbit, and the magnetotail is a gradually tapering part away from the Sun. Magnetotails generally are much larger than the magnetic objects associated with them and in Jupiter’s is around 489 million kilometres long, which is almost as far as Saturn and also means that the outermost moons of that planet actually pass through Jupiter’s magnetotail at times, and that the magnetospheres probably touch sometimes. Strictly speaking, magnetic fields have infinite range but after a while it gets silly.

Like Earth’s Van Allen belts of Apollo mission fame, further in towards the planet Jupiter traps charged particles, which are unfortunately where three of the four largest moons orbit. There is also a plasma tunnel, but this will be made clearer on a later date.

Jupiter has eighty moons. Sixty are less than ten kilometres across. I tend to think of both Jupiter and Saturn as like archipelagos of islands with a few large islands and multitudes of smaller ones. In Jupiter’s case, the moons are grouped into orbital zones with large gaps between them. I’ve already talked about Amalthea, one of the inner moons, and I’m not planning to plod through a massive long list of mostly tiny, boring and very similar moons, but they’re collectively of interest and the way they’re grouped is also significant.

The Galileans are the “big four”. Each of them is practically a planet in its own right, and they were also the first moons to be discovered orbiting another planet, by Galileo in 1609. Another astronomer, Marius, found them just one day later and he’s responsible for the names. These are also the first celestial bodies to be given names in written history. However, the Chinese astronomer 甘德 discovered either Ganymede or Callisto in 364 BCE, because they are bright enough to be visible to the naked eye of someone with good vision. All of them are brighter than Vega from here. The Galileans form an important rung on the ladder of establishing the scale of the system and Kepler’s laws of planetary motion. When they’re relatively nearby, that is, when Earth and Jupiter are on the same side of the Sun, it’s fairly easy to look through a telescope and time their movements, as in, the points when they’re furthest from Jupiter, when they pass behind and in front of the planet and emerge on the other sides, a total of two dozen events. Their relative distances can be measured using this observation because of their maximum visual distance from Jupiter, and this enables it to be observed that, like the planets with the Sun, the cube of their average distance is directly proportional to the square of the time taken to orbit, Kepler’s third law of planetary motion. Then, when Jupiter is on the other side of the Sun from us, there’s a delay in these observations of up to almost exactly a thousand seconds, which enables the width of our orbit to be calculated if one knows the speed of light. This in turn enables the scale of the orbits all observable planets in the Solar System to be calculated, and the difference between the periods of Jupiter’s Galilean moons and a hypothetical planet orbiting an object the mass of the Sun enables the mass of Jupiter compared to the Sun to be worked out as well. Working out the speed of light itself is a somewhat different problem. I’ve tried to do this but was stymied by fog. You need a clear day, a hill, a cogwheel, a mirror and a distant telescope.

The moons are organised into six groups. There are the inner moons, which include Amalthea, the Galileans Io, Europa, Ganymede and Callisto, and the Himalia, Ananke, Carme and Pasiphaë groups. These occur in bunches of orbits, but before I get to that I want to point out something else which is rarely mentioned: they changed the names of many of the moons in 1975. When I was a small child, before Pioneer 10 and 11 had been sent there, the names of the moons were completely different. This would’ve been in about 1972. By the early 1980s, the names of the outer moons had completely changed. The previous names were as follows:

• VI – Hestia
• X – Demeter
• VII – Hera
• XII – Adrastea
• XI – Pan
• VIII – Poseidon
• IX – Hades

The corresponding names now, in order, are: Himalia, Lysithea, Elara, Ananke, Carme, Pasiphaë and Sinope. Many more moons have been discovered since then. It’s all the more confusing because one of the inner moons is now named Adrastea. The scheme I was familiar with was apparently the 1955 proposal, which was used after a phase during which they were simply referred to by their Roman numerals, listed in order of discovery. There were also proposals in 1962 and 1973, and once again Adrastea is used, this time to refer to Himalia. The current names are the 1975 IAU version, and there is also Carl Sagan’s 1976 version. Nowadays, the moon names ending in E are retrograde – they orbit in the opposite direction from the majority of bodies in the Solar System – and prograde moons have names ending in A. There was also a tendency to choose names from the lovers of Zeus or Jupiter in Greek or Roman mythology, of which there are a very large number, so the supply was clearly considered almost inexhaustible. The view was also taken that irregular moons shouldn’t be named at all but just left with Roman numerals. Now that eighty moons are known, I suspect they’ve finally run out of lovers. The question arises in my mind of why there are no homosexual lovers since homophobia didn’t exist in the Greco-Roman world before the arrival of Christianity, but I think this is because one of the reasons Jupiter and Zeus had so many is so they could serve as the origin story for various beings seen as a mixture of the qualities of the two parents. There’s also an inconsistent tendency for the moons to be given names across the systems which start with the same letter, such as Hestia and Himalia, and Poseidon, Persephone and Pasiphaë. Up until the 1970s, there seemed little point in naming them since at that time they were simply rocks spinning round Jupiter without much being known about them, although Isaac Asimov does refer to them in his ‘Lucky Starr And The Moons Of Jupiter’, though by the numerals rather than the name.

The inner satellites are all small, but Amalthea is the biggest satellite after the Galileans. Himalia is only slightly smaller although it isn’t an inner satellite. I’ve never really got used to using the newer names by the way. There are four small inner moons. Metis is actually technically too close to hold together, which is appropriate since it’s named after a titaness who turned herself into a fly and was eaten by Zeus. Incidentally, if I’d written the sequel to ‘Replicas’ it would’ve included a planet called Metis as an important plot point, but sadly it was not to be. The real Metis is on the brink of being devoured by Jupiter and is also only ten kilometres across. The three other moons were discovered via the Voyager probes in 1979 and not named for quite some time after. The spacing of their orbits is similar in scale to that of the Galileans. Amalthea may have associated moonlets but they’re not confirmed, the “flashes” only having been detected once.

After the Galileans there’s a big gap, and to some extent Jupiter’s system reflects the shape of the Solar System here in that there are four smaller inner moons like the four smaller inner planets followed by four much larger moons like the gas giants, but unlike the Solar System Ganymede, the largest moon of all, and in fact the largest moon in the entire Solar System, is the third large body rather than the first, and there doesn’t seem to be anything corresponding to the asteroid belt. The pattern of distribution of moon sizes may be a guide to how other star systems form and the Galilean orbits are in harmony with each other. Callisto is somewhat separated from the others, making it easier to spot and reflecting something like the Bode-Titius Series with the spacing of the planets. However, after Callisto comes a big gap. There is one small moon, Themisto, discovered in 1975, orbiting about halfway across that gap, but wasn’t observed for long enough for its orbit to be established. It was lost for a quarter of a century, and none of the probes investigated it. It’s fairly common for small Solar System bodies to be lost and later found again.

The next bunch, of seven moons, includes the incredible Leda, which is absolutely tiny for a moon discovered and confirmed from Earth observations in the 1970s. It’s turned out to be somewhat bigger than originally thought, and was discovered by the extremely prolific “discoverer” Charles Kowal who also observed Themisto, in 1974. Kowal also discovered the centaur Chiron. This set of moons is tilted at 30° to the inner group and has more elliptical orbits, all of which line up with each other. These are between eleven and thirteen million kilometres from Jupiter.

There is then another gap, within which orbit Carpo and Valetudo, closer to the third group rather than orbiting in isolation like Themisto. Unlike the outer group, however, they orbit in the same direction as the inner moons. Valetudo is only one kilometre in diameter, like several other moons, making it joint smallest, although there will presumably be some differences in size. It’s also currently the smallest named moon. I don’t know if they’re going to bother naming the others of this size, but the asteroid Adonis was named and is only five hundred metres across, although it’s also a potentially hazardous asteroid so that may be why it got one.

The outermost group orbits backwards compared to the others and in fact compared to most other bodies in the Solar System, which generally orbit clockwise viewed from the South. Hence they all have names ending in E: Carme, Ananke, Pasiphaë and Sinopë, which apparently is supposed to have a diæresis over the E. Incidentally there’s a village in this county called Sinope and also a town in Turkey, probably named after the nymph in the latter case, and no longer spelt that way. By the time you get to the outermost group, the orbits are considerably perturbed by the Sun. There’s a concept called the Hill Sphere, which is the sphere within which a body’s gravitational influence is stronger than any others, generally a planet and its star. Jupiter’s is fifty-five million kilometres in diameter, so the outermost group of moons are close to its edge. The ellipses of their orbits are also lined up, but currently at close to right angles to the middle group.

Although Jupiter’s Hill Sphere is not as large as Neptune’s, which is the furthest known large planet from the Sun and so has more elbow room despite its much smaller mass, Jupiter is more likely to sweep bodies up into its. This is because it only takes a dozen years to orbit the Sun compared to Neptune’s more than a gross, and is doing so much faster and in a more crowded region of the system.

The Solar System has jokingly been described as consisting of the Sun, Jupiter and assorted débris. Jupiter, although far less massive than the Sun, has around two and a half times the mass of all the other known bodies in the system put together.

There are many more things to say about Jupiter and its moons, but these will be about the planet and the bodies themselves, so for now I’m going to knock this on the head and publish it.

The World Ceres

Title nicked from Asimov.

On the first day of the nineteenth century CE, the astronomer Giuseppe Piazzi pointed his telescope at an area of sky in the hope that Bode’s Law wouldn’t fail him, and indeed found the first independently-orbiting body within the orbit of Saturn since ancient times. This was in spite of an organised posse of astronomers, the “Celestial Police”, searching the heavens for such a planet. They were later to find more, but Piazzi, who had actually been considered for membership of this group, beat them to it. This was the world later to become known as Ceres.

Bode’s Law is the rather unfairly titled principle which appears to determine the distances of the planets from the Sun. It was actually first arrived at by Johann Titius some time before. It uses the sequence 0, 3, 6, 12, 24, . . . , to each of which four is added, giving 4, 7, 10, 16, 28, and has been fairly successful in predicting the positions of various planets. It was popularised by Johann Bode, hence the name. The units amount in this case to tenths of an AU, which is the distance between Earth and the Sun, as is seen in Earth’s position in this series at 10. The series isn’t perfect. For instance, it’s anomalous that it starts at zero and Uranus doesn’t fit, although Neptune does. Nonetheless, astronomers noticed there seemed to be a gap at 28. Mars is 1.524 AU from the Sun on average, with an aphelion of 1.666, whereas Jupiter averages out at 5.204. Astronomers used the sequence as evidence for another planet, and they found it.

However, the planet they found was rather odd compared to the others known at the time. The smallest known planet in the eighteenth century was Mercury, now known to have a diameter of 4 879 kilometres. Ceres is much smaller than this at 946 kilometres. During my lifetime this figure has been revised several times, so I imagine it was different in the early nineteenth century too, but in astronomy books at the time, Ceres is clearly shown as much smaller than the other known planets, yet still acknowledged as one, before the asteroids were discovered.

Over the next few years, a number of other bodies were found between Mars and Jupiter, and the planets were split into the categories of major and minor planets to account for them. Ceres was relegated to the status of a minor planet or asteroid for a long time, up until the decision to redefine planets in 2006 as mentioned here, at which point it was put in the same category as Pluto, a “dwarf planet”. As I’ve said before, I’ve never really understood why there needed to be such a category when that of “minor planet” already existed, but it did at least put Ceres in the same pigeonhole as Pluto, which was some kind of progress. It’s an interesting history though, because it means its tale with us began as a planet, stopped being one and then became one again. Also, in the light of what I’ve said previously, nowadays it could even simply be seen as a planet.

Ceres is not like the asteroids, even though it orbits among them. It conforms to the second 2006 criterion of planethood in being round due to its gravity. No other asteroid is so close to being spherical and the margin is actually quite sharp. The next closest seems to be Hygeia. Taking all known bodies in the system into consideration, the smallest round one is Mimas, which orbits Saturn and has a diameter of 396 kilometres, although it has an enormous crater which prevents it from being perfectly round. It isn’t “lumpy” though. Hygeia is actually larger than Mimas with a diameter of 444 kilometres, and is in fact a candidate dwarf planet in itself. There could be much smaller asteroids which are round, but if so this wouldn’t be due to their gravity.

The planet, for that’s what it is really, is the smallest in the system which orbits the Sun independently, but it also contains the bulk of the mass of all bodies between Mars and Jupiter, at about 30%. This means that even if the hypothesis about a lost, shattered planet there had been correct, or if Jupiter was in a different place and the mass of the asteroid belt had been able to assemble itself into one, it would still be smaller than Mercury or even Cynthia. Because it’s long been dismissed as an asteroid, Ceres has occupied a kind of second-class place in the system for a long time and consequently I for one, and presumably most other people who have learned abut these things, can’t easily reel off a list of statistics and facts about the planet as I would with, say, Uranus or even Pluto. I know its day lasts nine and a bit hours, that it has a very thin atmosphere indeed, not really even worth mentioning, but I don’t know its largest craters, axial tilt, how long it takes to orbit the Sun, highest peaks, climate or any unusual features. I do know that it has more water ice as part of its actual internal structure near one of the poles and that it has some water ice on its surface.

The distance from the Sun is kind of “unusual”. In fact it isn’t unusual at all as the zone Ceres occupies in its orbit is the most crowded of any in the system. However, because we haven’t tended to think of Ceres as a planet, and to be fair it is still something of an outlier as far as planets directly orbiting the Sun are concerned, we haven’t considered what happens at this distance. The main consequence is that it has an unusual range of surface temperature, between -163 and -38°C, which means that at its warmest its temperature overlaps with Earth’s. In other star systems there are probably larger planets in this kind of orbit because of other characteristics being different, such as no giant planets or giant planets in different positions, but for our system this is notable for being intermediate between the coldest (on average) terrestrial planet and the warmest gas giant. If it had the same atmospheric pressure as Earth, Ceres would be able to have liquid ammonia on its surface which could both freeze and evaporate, and the chances are there’d be an ammonia cycle like our own water cycle, along with rivers, lakes, rain and even snow and glaciers. However, in reality there’s practically no atmosphere. Even so, ammonia is rich on the surface and participates in the planet’s geochemistry, which suggests that it originated far out in the outer system where the compound is more abundant. There are clays rich in ammonia and ammonia salts in some of the craters. There is also the intriguing ammonium ion, NH₄⁺. This is distinctive in both bearing a single positive charge and being about the same size as some alkali metal ions, meaning that it behaves as if it’s a metal ion like sodium in sodium chloride, and can even form amalgam with mercury and sodium like solid metallic elements. In other words, it can form into metallic alloys even though it isn’t itself a metal. Due to all this, the geology of even the surface of Ceres is unique, at least for the more reachable part of the system. I may be wrong about this but I think of it as a clay-covered place, except that instead of water making it moist, ammonia does that job instead, and also unlike water (although the hydronium ion is common in the Universe, which is to water as ammonium is to water) in that it behaves a little like an alkali metal.

The asteroid belt divides the five inner terrestrial from the five traditional outer planets (gas giants plus Pluto) of the outer. Hence Ceres can be thought of as the middle planet of the Solar System, or to put it another way, central to it. This is not literally true because as the Titius-Bode Series shows, the planets are each almost double the distance of their predecessors from the Sun counting outward. This means that its composition and temperature are intermediate. It may or may not have a global ocean under its crust. This may have existed but will now have frozen. It would be possible to detect because it would be salty and this would make it detectably magnetically.

There is a single remaining extinct cryovolcano on the surface called Ahuna Mons, which is five kilometres high. At some point I will need to address what counts as height on planets without bodies of liquid on their surfaces. In this case there’s a clearly visible crater next to the mountain, Occator Crater, and it wouldn’t be sensible to assess its height from the bottom of that crater although it might influence its structural integrity. There are white streaks on the slopes like lava flows, and also like the white patches elsewhere on Ceres, all of which are probably salt. Incidentally, although “salt” brings sodium chloride to mind, I can’t find out whether this is the salt in question or whether it’s ammonium chloride, which is also white, or something else. It could be a mixture, but that’s my speculation. There are also possible traces of smaller volcanoes. There’s a concentration of mass about thirty kilometres below it, which suggests it was formed from a plume of mud rising from the mantle (which was probably watery). There’s also sodium carbonate (washing soda) on the slopes, which is found on Earth in desert regions as the mineral natron, used in the Egyptian mummifying process and to make glass. Ahuna is almost exactly opposite to the largest impact crater, Kerwan, suggesting that it may have resulted from shock waves moving around the planet and concentrating on the other side, where they fractured the crust. This happens a lot with large impacts. For instance, Caloris Planitia on Mercury is opposite so-called “chaotic terrain” on the other side, and in fact this is making me wonder right now what was opposite Chicxulub when the impactor hit, killing the larger dinosaurs.

Occator, next to Ahuna, has the largest concentration of bright spots. I have to say, looking at images of all the large bodies in the Solar System, Ceres is distinctive in having small white areas fairly sparsely distributed across its surface. These have been named faculæ, meaning “little torches” in Latin, a name first used to refer to bright spots on the Sun’s photosphere. They’re near ammonia-rich clays and are rich in magnesium sulphate, which is Epsom salt, so the whole planet has a kind of domestic chemical theme going on. These are on a hill in the centre of the crater called Cerealia Tholus, and at this point it’s worthwhile mentioning the name. Ceres is named after the Roman goddess of arable farming, after whom cereals are named. Ceres is known substantially for her daughter Proserpina, more often known by her Greek name Persephone, who was forced into marrying Pluto and living in the underworld, but finding that she could return provided she didn’t eat any food there. However, she ate three pomegranate pips and is therefore condemned to spending a third of the year there. Ceres mourns this by causing winter, and celebrates her return to the upper world with spring. The Greek equivalent of Ceres is Demeter, after whom a moon of Jupiter is called although this was renamed in 1975. Thereby hangs a tale, incidentally: Jupiter’s smaller moons all got renamed in the mid-’70s. The whole domestic flavour of the place is once again confirmed by the mention of cereal. This is a planet made of washing soda, ceramic (kind of) and Epsom salts named after the goddess of cereal! The rare earth metal cerium, discovered two years later and now used in lighter flints and the subject of an essay by Primo Levi, is named after the planet, rather like uranium, neptunium and plutonium.

Occator is unusual in having a central hill. This is normal on many craters on other bodies, but Cerean craters tend just to have dents in the middle. The largest crater is the previously mentioned Kerwan, one hundred and eighty kilometres in diameter. It isn’t clear if it had a central peak because a smaller impact has created a crater where that would be. It’s named after the Hopi cereal nymph, this time for sweetcorn.

Zooming out a bit and treating it as a planet like any other, as opposed to the asteroid it was formerly presumed to be, Ceres averages 2.77 AU from the Sun, approaches it most closely at 2.55 and has an aphelion of 2.98, which makes its orbit slightly less elliptical than Mars’s at 0.0785. It takes somewhat over four and a half years to orbit the Sun and is inclined 10.6° to the ecliptic, which is greater than any other planet out to Neptune unless you count the moons of Uranus (see the post on planet definitions if you don’t get why I’m calling them planets rather than just moons), though less than twice that of Mercury. Looking at the three planets Earth, Mars and Ceres as a, well, series, there is a trend of reducing size. Mars bucks the apparent trend of increase in size up to Jupiter followed by a decrease in size out to Pluto, but if Ceres is included a new possible tendency is revealed, also reflected in reducing density as Earth is over five times as dense as water, Mars and Cynthia around three times as dense and Ceres a little over twice as dense. This may just be playing with numbers, but it’s also possible that Earth hogged all the material, only leaving a few leftovers for the planets closer to Jupiter’s orbit. As for density, the closer planets to the Sun would have been warmer when they formed and this seems to have caused the icier components, or simply those with higher melting and boiling points, to evaporate off. However, Ceres seems to have formed in the outer system. It has an axial tilt of only 4°, so ironically the planet named after a goddess closely associated with the seasons has no seasons of its own. Surface gravity is less that three percent of ours, so if I went there I’d somewhat exceed my birthweight but only because I was small for dates.

Looking at the planet and knowing that most of what I’m seeing is clay puts me in mind of the idea that Ceres has an affinity with the various planets which show up in claymation shows. I can imagine its appearance turning up on someting by Aardman Animation, and it makes me wonder what the Clangers planet was originally made of. However, this is largely in my mind. It’s all very well looking at an image of Ahuna Mons or the planet as a whole in full knowledge that it’s mostly salty clay and seeing it like that, but on the other hand many of the craters are æons old and don’t seem to have sagged in all that time, although they do lack the central mounts found elsewhere. It may be more accurate to think of the planet’s surface as being made of frozen clay rich in ammonia, and it also isn’t clear what clay’s like if it’s mixed with liquid ammonia and well below freezing point as opposed to the stuff we make pots out of. I think Ceres may be the kind of place where our intuitions based on how things are here, or even in the outer system, may mislead us. That said, the edges of the craters are less well-defined and the floors are smoother, and when it was actually being hit by something it would presumably have melted or boiled the material, so at that point maybe it does behave like clay or go through a phase of clay as we know it as it cools down.

Although it doesn’t have an iron core, the planet is likely to have a core high in metals, but also in silicate rocks. The pressure on it will be far lower than on Earth’s core. Our planet is close to 6 371 kilometres in radius, more than twice as dense as Ceres and has thirty times the gravity. Put all of those together and it makes the pressure at the core something like (and these are back of the envelope calculations) what it would be only ten kilometres down in our own crust, or even less. This is only the level of an ocean trench and only a few times deeper than the deepest mines. Consequently the settling out effect of the originally molten planet is milder and not so influenced by pressures beyond easy imaginings. Outside that core is a mantle of silicate rock which may have squeezed out the water and ammonia, or they could have separated out due to being lighter. Above that is a probably frozen solid ocean, and finally on the surface lies the clay-rich crust with salty deposits. All this notwithstanding, it’s also been accurately described as “icy, wet and dark”, i.e. it has a dark surface. It isn’t particularly dark as far as sunlight is concerned.

There are several more ways in which Ceres is special. It’s a survivor from the early Solar System, in that it’s a protoplanet. Near the beginning, there would’ve been hundreds of small planets like this, large enough to undergo interior melting, which mainly happens due to radioactivity, and therefore stratification like Ceres has, but many of them would have collided with each other and stuck together, possibly been thrown out of the system entirely by close encounters with others accelerating their movement. Along with Vesta, which is more battered and smaller, Ceres is a surviving relic from shortly after the Sun formed. It’s also the closest dwarf planet to Earth, the first dwarf planet to be visited by a space probe, the first time a space probe had orbited two bodies on its mission and the largest body except Pluto-Charon not to have been visited up until 2015.

The spacecraft which visited it is also quite interesting. It’s called Dawn, and was actually launched at dawn one day in 2007. It used Mars to accelerate its path and visited and orbited Vesta, also a first, in May 2011. Vesta is interesting in itself, and I’ll be covering that soon as well. It then left Vesta and made its way to Ceres, becoming the first spacecraft to actually orbit two bodies in the Solar System unless you count the orbits made of Earth before some spaceships have headed off into the void. It’s still orbiting Ceres but its mission is now over. Dawn was also the first craft to use ion drive, an idea for a very efficient but slowly accelerating engine which can accelerate vehicles so fast they could cover the distance between us and Cynthia in less than two hours, without using gravitational assist, which is the usual reason space probes are accelerated to this velocity and beyond.

There is plenty more to say about Ceres, but I want to finish as I started: with the pun. Isaac Asimov used to be very fixated on puns, and several of his short stories were only written to make puns. In the case of his article ‘The World Ceres’, published in 1972, he may have been primarily motivated to write it just because he could use a good pun in the title. I have read it but I don’t remember how much detail he went into. It doesn’t seem likely that much was known about it at the time, but I may be wrong. It might be interesting to compare factual articles on astronomy before and after they were visited by probes. For Ceres, this period was a lot longer than usual, but also occurred only 206 years after it was discovered, which is pretty good going.

The Cosmic Lychee

Spelling the fruit that way makes me twitchy, but apparently that’s how it’s spelt, rather than “litchi”. Of course, ultimately it’s spelt “荔枝”. I could go all herbal on you here but instead I’ll just sum up my issue with it. Lychees are ultimately disappointing and hard work. They have massive stones in the middle and fiddly peel on the outside plus a really thin layer of pulp. The stone is poisonous, causing encephalopathy, so you don’t get any benefit from that and there’s just rather annoying in the end.

If a particular planet was a fruit, what fruit would it be? I suppose, and it may surprise you that I haven’t thought much about this, that the rocky inner planets would mostly be like nuts, being hard and woody, and the gas giants more like succulent fruits with maybe a small stone in the middle. There is a memory palace including planetary associations with herbs and sometimes fruit, so for example orange is a solar fruit and bananas, being crescent-shaped, are lunar, but this is not quite what I mean. The Chinese names for the planets use the five element system, so they too have a kind of taxonomy. In this system, Venus is “金星”, which literally means “gold star” or “metal star”, metal being one of the elements. Jupiter is “木星”, meaning “wood star”, making it sound more like a nut, so my view of lychees is not backed up by traditional Chinese cosmology. Oh dear.

On the whole, the major solar planets are presented as falling into two main types: terrestrial and gas giants. Terrestrial planets are mainly rocky and occupy the inner system. They’re denser, smaller and warmer than the other local planets, known as gas giants. Gas giants are mainly gas, although further inside this may be compressed to liquid or metallic form, and are much larger, colder and less dense. A further subdivision is sometimes made distinguishing Uranus and Neptune, the “ice giants”, from the two “gas giants” Jupiter and Saturn. The two classes are separated by an asteroid belt. The markèd division therebetwixt may be somewhat blurred in other star systems, since the most widespread type of planet in the Galaxy seems to be one which is almost halfway between the size of Earth and Neptune, which seems to have no analogue orbiting the Sun, so the separation may be artificial and based on local experience rather than them being natural kinds (if natural kinds exist).

For most of the inner planets, this division works fairly well. Mercury, Cynthia (I’m not going to keep explaining) and Mars are all solid planets made largely of rock with thin or almost non-existent atmospheres. Earth is a slight deviation from this pattern. It’s a dense rocky planet with a fairly dense atmosphere and is unique among such planets in having large persistent bodies of water and exposed solid surfaces.

Then there’s Venus. This is usually understood as a rocky planet with a very dense atmosphere, and a hellishly hot solid surface. That’s fine in terms of what it can be described as, but in context this may be slightly different. Back in the day, it used to be thought that the gas giants all had very large rocky cores, and they do have rocky cores, but they were seen as essentially solid planets with deep atmospheres shrouded in cloud. This is a rather “Gaiacentric” way of looking at them. Even back then it was considered a tall order to send any kind of lander onto the purported rocky surface of the “real” Jupiter or whitherever, but nowadays that’s recognised as such a feat of engineering as to be basically impossible. Hence we do have planets in our solar system which are mainly gas and on which few people would contemplate actually landing. At the same time, even the terrestrial planets are very hot on the inside and we haven’t even succeeded in penetrating far into Earth’s surface. It might be dangerous to do so, creating a new active volcano at the site where the tunnel was dug. Jupiter is similar, only more so, with its internal temperature rising far above that of the Sun’s photosphere.

My contention is that we’ve got Venus all wrong. To an extent this is just playing with ideas, but there are sound reasons for thinking of the planet differently. The Soviet Union’s record for sending landers to the surface of Venus was impressive. However, were they really sending them to the surface? Jupiter’s “surface” is effectively the cloud tops. Another way of putting this is that the surface of a planet is where it becomes opaque from space. This is only provisional as it leaves Earth’s oceans in an ambiguous position. Considering Venus in those terms, its surface is the cloud tops and the landers are actually penetrating into the interior of the planet. Unsurprisingly, just as a probe sent into the magma under our crust would meet with a swift demise, so do the landers on Venus, just as Cassini undoubtedly did when it fell into Saturn’s atmosphere, also known as Saturn. Hence Venus is a lychee. It has a big stone in the middle and a thin pulp, by contrast with the thick pulps of the gas giants. I imagine the name has already been taken, but I think of Venus as a “gas dwarf”.

This is not mere tinkering with ideas. Considered as a planet with a gaseous surface, Venus immediately starts to look a lot nicer than the usual hellscape it’s portrayed as. It’s probably well-known that there’s a level in its atmosphere where temperature and pressure are both close to what they are at sea level on Earth. The clouds start seventy kilometres above the solid surface, although there’s a haze extending for another fifteen kilometres or so. Both Venus and Earth happen to be at the same temperature at that level, although the pressure on Venus there is similar to that on the surface of Mars. The layer in which all the weather happens is known as the troposphere on most worlds, and this level is thoroughly within its troposphere, unlike Earth’s which has a ceiling averaging at thirteen kilometres (like Earth itself, out troposphere is squashed at the poles and protrudes at the equator, so the height varies). The altitude where it matches Earth’s surface is towards the base of the clouds at fifty kilometres up. There is then another haze layer down to thirty kilometres, meaning that the clouds in the Venusian atmosphere actually start above the level of our own cloud tops, with a few anomalous exceptions. The pressure of the atmosphere at the mean solid surface level is about the same as the water pressure a kilometre down in Earth’s oceans. Here again is the theme of the bottom of the ocean, like the bottom of Venus’s atmosphere, being more akin to the interior of the planet than its surface.

The atmosphere of Venus is almost all carbon dioxide, which is why it has such an extreme greenhouse effect, having the hottest solid surface in the inner system, including Mercury. Various processes could have contributed to this outcome, although it serves as an awful warning to us of what could go wrong. There was probably a time in the past when Venus had liquid water on its surface. It would have been below boiling point at that time, but there still would’ve been a lot of evaporation, and water vapour is a powerful greenhouse gas, far more so than carbon dioxide. Another cause may have been the exposure of carbonate rocks to long periods of sunlight, which baked the carbon dioxide out of them into the atmosphere. All of this was triggered by the gradually climbing heat of the Sun in the early history of the planet, and the presence of water on a planet which at that time was very similar indeed to Earth at the same time suggests that life may have been present. However, I’m resisting the urge to turn everything here into stuff about the likelihood of life in various ways, so I won’t be discussing that. Although Brian Cox’s ‘The Planets’ was fun, one of the irritating things about it was that he tended to focus very much on the issue of liquid water and the possibility of life emerging throughout the Solar System, and I’m not going to do that. It gets a bit repetitive. Therefore, with respect to Venus I’m just going to say: carbonyl sulphide, clouds absorbing ultraviolet life, phosphine. There, all done.

Venus has three times our atmospheric nitrogen. If much of Earth’s nitrogen wasn’t fixed and/or in living things, we would also have two or three times the amount we have, except that if it wasn’t there wouldn’t be a “we”, so “we” probably started out with about the same amounts. The other gases in its atmosphere are carbon monoxide, hydrogen chloride and hydrogen fluoride, although in quite small amounts in the last two cases. Nonetheless it might make a quite good toilet cleaner. There’s a fair bit of water vapour, at about one percent, bearing in mind that that’s one percent of an atmosphere ninety times thicker than ours, so it’s more like the equivalent of the whole of our atmosphere in just water, which is about a gigatonne.

The clouds in the Venusian atmosphere rotate around the planet about once every four days. Since its day is supposed to last 243 days, once again this raises the issue of which bit is the surface, and again it makes a lot more sense to have a planet rotating every four days than nearly nine months. This makes the atmosphere the fastest rotating compared to its planet in the Solar System.

The solid surface of the planet is worth comparing to Earth’s. Our own surface is a series of plateaux with fold mountain ranges, is shaped by water erosion and plate tectonics, and is largely abyssal plains with central ridges and trenches near the continents. Venus is not entirely similar, but it does have plateaux which could be thought of as continents. This is a map of how its terrain would look like with water:

A decision must be made in such maps regarding where to put sea level. This seems to be Venus with the same areal water cover as Earth, but that’s neither necessarily the same as all the actual water vapour in its atmosphere condensing out or the scaled amount of surface water on Earth, because the topography of Venus is very different from ours. It’s also Venus with water but no water erosion, and the colour scheme indicates the likes of mountains with snow, vegetation and more barren ground, which presumes to know the climatic profile of the planet. It’s also difficult to be objective about this, in that it may or may not be a typical Earth-like world. My impression is that it has many more islands than Earth would have, although the part of our planet which constitutes a single large plate, the Pacific, is also studded with islands in a somewhat similar way.

Without water, i.e. as it actually is, the solid surface of Venus looks like this:

Thinking in terrestrial terms, the two major continents are Ishtar and Aphrodite, with a smaller group arrayed north-south to the “west”. I should point out here that compass directions on Venus are a little confusing because the planet rotates backwards compared to all the other inner planets, and I have to confess I don’t actually know which way up this map is compared to Earth maps with North at the top. On that subject, Venus lacks a global magnetic field of its own, although it does have the bow shock and limited magnetosphere resulting from the solar wind. It isn’t known why Venus lacks a magnetic field, but it may be because its mantle doesn’t convect much, and unlike us, it lacks any companion to raise tides in its core. This is a striking difference between the two planets which doesn’t seem to be easily explained in the other ways.

Before the ’60s, Venus beneath the clouds was utterly unknown and people, including scientists, made all sorts of projections onto it. Carl Sagan once joked about proceeding from the premise of not being able to see the solid surface to the conclusion that it was covered in steamy jungles and dinosaurs, which was a fairly popular view. This is partly influenced by the idea that the closer planets are to the Sun, the younger they are, which is not in fact so. Various views have been taken regarding the nature of Venus historically, all of which are much more interesting than the reality. It was thought to be an ocean planet with soda water seas (I kid you not) at one point, prior to which it had been considered to be a more Earth-like ocean planet as with Asimov’s ‘The Oceans Of Venus’. People often talk about the shock to society which would ensue if life was incontrovertibly discovered elsewhere, particularly intelligent life but still just life, as in the smallest, simplest bacterium, but this could be overestimated because there was a time when life on Venus and Mars was practically assumed, and it had no major impact on humanity. Have we maybe changed in this respect?

Seventy percent of Venus’s solid surface is a low, rolling plain. There are a few basins but they’re quite rare, although depressions are common at about twenty percent. Ten percent are highlands, which can be thought of as continents although there is no continental drift on Venus. Height of topography is defined as deviation from mean radius, which is easier on Venus than Earth. Earth is squashed at the poles and bulges at the Equator, but Venus, uniquely among solar planets, is almost exactly spherical. This regular tendency is reflected in its orbit, which is also the most circular of the lot. In a way this can be considered appropriately beautiful.

Ishtar is about the same size as Australia and averages three kilometres above the mean radius. It’s a volcanic region. Aphrodite is Afrika-sized and has mountains at the eastern and western ends with a low-lying area between them. It lies along the equator. The third upland area is called Beta and has two large shield volcanoes, and finally there’s Alpha, which is somewhat similar to the Martian area Tharsis.

There is also a pair of rift valleys, Diana and Dalí. The former is up to 280 kilometres wide and somewhat like the Valles Marineris on Mars. I understand that structure to result from the crust fracturing due to the weight of the Tharsis shield volcanoes, because Mars has no continental drift, which leads to more and more lava building up and solidifying until it weighs the crust down. I don’t know if Venus also has this phenomenon.

Ishtar has a plateau referred to as the Maxwell Mountains. This has a mountain called Skadi which is 10.7 kilometres high, the highest point on Venusian land. This makes it higher than any terrestrial mountain although it should be borne in mind that those are measured from sea level and the difference between the bottom of the Marianas Trench and Mount Everest is over twenty kilometres. However, Diana is the lowest point on Venus and is only 2.9 kilometres below the mean, so Skadi can still be thought of as higher than any earthly peak, although the difference between the average depth of our ocean floors and the highest mountains is about the same. This suggests that the material from which our crusts are made is similar, as it will tend to collapse at about the same height, bearing in mind the slightly lower gravity on the inner planet.

At ground level, the temperature of Venus doesn’t vary with latitude or day and night because the atmosphere is so dense that it carries heat all over the planet equally. Polar regions are nothing other than that. There are no magnetic poles and the temperature is no lower than anywhere else. That said, the temperature of the atmosphere does vary and is very cold at the poles. It reaches -157°C and there are atmospheric waves. No, I don’t know what that means but apparently there are.

The Venusian clouds themselves are largely composed of sulphuric acid droplets. No mountains are able to reach anywhere near even the bottoms of the haze below the clouds. However, there is sulphide frost at the top of some of the mountains, namely sulphides of bismuth, lead and iron – bismuthinitem galena and fool’s gold. This can be detected by RADAR from Earth as it reflects like a metallic surface. There seems to be lightning, but it isn’t clear if it’s to do with volcanoes or clouds. The Soviet Venera 11 lander which landed on Christmas Day 1978 had something akin to a microphone on board and detected an eighty-two decibel noise after landing, which may have been thunder. The density of the atmosphere would have made the noise a lot louder than thunder on Earth. There’s also a phenomenon called “virga” – I don’t know how well-known this word is. This also happens on Earth, but on Venus it’s the only form of precipitation unless you count the frost. Virga is rain which evaporates before it reaches the ground, although on Venus it’s sulphuric acid rather than water. This happens because the temperatures get way past boiling point high above the ground. The sulphuric acid forms when ultraviolet light from the Sun separates carbon dioxide into carbon monoxide and monatomic oxygen, the latter whereof then combines with sulphur dioxide, to become sulphur trioxide, and water vapour.

There is said to be a glow on the night side, referred to as “ashen light”. It isn’t even known to exist, and was initially attributed to phosphorescent organisms in the ocean which was thought to cover the planet, and I think to aurora. Various suggestions have been made. Because Venus has no real magnetosphere, it could be solar radiation directly stimulating the atmosphere, since it’s able to reach cloud level at least. Alternatively, it might be lightning leaping between clouds. Although it’s been spotted since the seventeenth century by astronomers as eminent as Patrick Moore, it isn’t universally accepted to exist as no spacecraft have ever detected it despite some having instruments for that purpose. It used to be thought that the “black drop effect” seen when Venus crosses the Sun’s disc, where the planet starts off with a narrowing tail towards the limb of the star, was due to the presence of a substantial atmosphere, but it’s now been seen with Mercury too, so it seems to be some kind of optical illusion instead.

There are some relatively famous pictures of Venus taken in the ultraviolet. This one was taken by Mariner 10 in February 1974. Since ultraviolet can’t really be considered visible to people with normal vision (there are circumstances in which it is but let’s not split hairs), this is of necessity false colour, and making the more absorbent regions blue makes Venus look more like Earth. Nonetheless, someone who could see ultraviolet would probably see the two planets as more alike than most people can. In the visible range, Venus is the most reflective planet and can actually cast shadows of its own in some circumstances. In any event, the swirls of cloud seen in this image are typical of Venus in their general global distribution. There are cloud belts a little like Jupiter’s. The relatively homogenous nature of the terrain beneath them and their height means that there are neither chains of clouds as formed here by peaks and islands nor the variation due different conditions over sea and land. On Venus the most distinctive feature is probably the “sideways” V-shape with its point near the equator although the brightness of the poles is notable too. There is a C-shaped cloud centred on the north pole rising up to fifteen kilometres above the other cloud tops. A figure-of-eight-shaped pair of hurricanes was thought to be another permanent feature of the north pole but a similar one disappeared from the south pole, and since the planet is highly symmetrical this suggests the same could happen in the north.

There are east to west winds whose speed gradually increases with height. At ground level they’re at about one metre a second and at the cloud tops around a hundred times faster. This is tantamount to the planet’s atmosphere rotating much faster than the solid surface. The density of the atmosphere further down makes it less penetrable to sunlight than might be expected, as it’s about a tenth that of water and is almost a liquid in a sense. For instance, it’s very buoyant and has waves as well as winds.

Since it was discovered that Venus is as harsh as it is below the clouds, suggestions have been made as to how it might be made more clement for life as we know it. This process is known as terraforming. An early suggestion was to seed the clouds with algæ. Recent suspicions that there might be life in those clouds already raise a major ethical question, as this could, for all we know, be the only other life in the Universe, and clearly such algæ would have to be genetically modified. Another possibility is to place a shield over the Sun at the L1 point with the Sun, ultimately causing the atmosphere to freeze, but that would then require the removal of a prodigious amount of dry ice. The same suggestion has been made regarding Earth to counteract global warming, and I included it in my novel ‘1934’. A further possibility is to steer some of the countless icy asteroids and comets to crash into Venus, where they will melt and provide water for oceans. The lower estimate for how long it would take has been two centuries, although I got the impression that was creeping up.

Since I don’t believe we will ever do it, to me it’s an abstract ethical question, which first seems to have been considered by Olaf Stapledon in 1930. This, actually, is interesting because of his connections with C S Lewis. In the scenario he described, in ‘Last And First Men’, Earth was threatened, and ultimately rendered uninhabitable, by our satellite crashing into us, which he portrayed as connected to human spiritual enlightenment, possibly Humanity discovers Venus to be inhabited by intelligent life whose expectations, however, are limited due to the fact that their metabolism depends on radioactive isotopes whose supply is limited. Humankind ultimately decides that they are carrying the torch for sentience furthest in the solar system and make an apparently rational decision to terraform Venus, which wipes out the native life. The inner life of the Venerians is too alien for humans to empathise or understand, but they’re confronted with this problem, as he states it: “what right had man [sic] to interfere in a world already possessed by beings who were obviously intelligent, even though their mental life was incomprehensible to man?” As a result of their genocide, humanity ultimately falls from intellectual grace and becomes eclipsed for millions of years in total despair.

Here comes the interesting part! C S Lewis was persuaded, apparently by this passage, that Stapledon was basically evil and a “devil worshipper” (his words, not mine). In a letter to Arthur C Clarke, he said:

 a race devoted to the increase of it own forces & technology with complete indifference to either does seem to me a cancer in the universe.

This is in reference to ‘Star Maker’ but seems to me to be clearly influenced by the attitude he read Stapledon as taking towards the Venerians. He is also said to have based a major human antagonist in ‘That Hideous Strength’ on Stapledon. I have to confess that I didn’t get very far with ‘That Hideous Strength’. That said, Lewis also acknowledges Stapledon’s influence on the novel in a positive way, saying:

 Mr. Stapledon is so rich in invention that he can well afford to lend, and I admire his invention (though not his philosophy) so much that I should feel no shame to borrow.

I don’t want to turn this into a discussion of the relative merits of Stapledon’s and Lewis’s philosophies of life, but this does vivify the ethical question of the terraforming of Venus. It’s unlikely in the extreme that there is in fact intelligent life there, and arguable whether there’s life there at all. If there isn’t, there’s still the question of whether it would constitute a form of vandalism to do this to the planet. The motives are relevant here. For instance, is the idea to create some kind of utopia, and is that even doable? Would it be anarchistic, governmental or corporate-based, or perhaps something else we can’t currently envisage? Would the presence of a better world influence ours au Guin’s Anarres and Urras might impinge on each other? Do we perhaps even have the duty to do it given the way we treat our own planet in order to increase the chances that life will go on after us? As I said, though, the chances are it’ll never happen.

I’m aware that I haven’t mentioned ‘Perelandra’ here. Nor have I read it, so I think I’ll leave it at that.

Every Side Up

A couple of posts ago I mentioned what I understand to be the anomalous nature of not having a widely-accepted proper name for that thing in the sky which lights up the night, looks about the same size as the Sun and is often shown as a crescent in children’s books: the so-called “Moon”. Well, it turns out this is just the start, and relates to a number of other ruminations I’ve had over the years. Although we intellectually accept that we are on a tiny blue speck orbiting the proverbially unregarded yellow star in the Perseus-Carina-Cygnus Arm of the Milky Way, which is in turn just one of countless other galaxies like grains of sand, as Brian Aldiss once put it, emotionally we tend still to operate day to day by the “sandwich” model of the Universe, where we live on a flat surface with the ground underneath us, the sea off somewhere across the way and the sky above us, with the Sun and Cynthia rising and setting above us. But is it psychologically healthy to do this? Is it a sign of having well-adjusted brains? Or, should I say, being well-adjusted brains, if we are indeed our brains.

I’ll start with Cynthia. As I mentioned the other day, I chose to call her Cynthia because that is in fact the name of one of the Greek goddesses associated with the big round hunk of rock some astronauts went to to prove a point about capitalism in a rather heavily government-assisted program half a century ago. Other Western options include Diana, Artemis and Selene, and there are wider possibilities which it might be only fair to include considering the heavy Greco-Roman bias for the names of the larger planets, moons and asteroids. Other sky lores are available. Such deities include Ge, Coyolxauhqui, Meztli, Tecciztecatl, Aucimalgen, Mama Killa, Qango, Tsuki Yomi, I mean I could go on, there are lots of course. The Latin word “luna” and its descendants, found in Romance languages and for some reason apparently Russian as well, is itself a euphemism for the earlier “mensis”, which became too strongly associated with menstruation and presumably made it sound to them that there was a “period” in the sky, which considering the taboos many cultures have around it led them just to call it “the light”, “lumina”, which then became “luna”. The Etruscan goddess is Tiur, with other names, and it seems to me that they could just have called Cynthia after that, but they didn’t. There are also kennings, which I’ve considered using directly or as an inspiration, but old Germanic literature doesn’t seem to have much occasion for mentioning the big light in the night sky for some reason. The options there seem to be “moon-wheel”, which is obviously a bit unsuitable but is a nice idea, conjuring up a rotating half-light, half-dark sphere viewed from its equator, “year-counter”, “waxer” and “waner”. I suppose I could’ve called it “sky-rabbit”, but the word “sky” is problematic too. In order to avoid the rather jarring and eccentric “Cynthia”, I do try to circumlocute references to her.

A couple of you have said it all seems a bit unnecessary, and I have sympathy with that idea. That said, calling our moon something other than “Moon” asserts her individuality. Just on the question of gender, although moon goddesses are more common than moon gods, the Old English word “mona” is actually masculine and “sunne” feminine. Once again, sun gods are more common than sun goddesses, such as Apollo, Helios, Ra and Sol Invictus. It’s not unusual for Germanic folk to get things the “wrong” way round, such as using nights instead of days to count time (“fortnight”), winters instead of summers on a longer timescale and considering the tail rather than the head as the “start” of an animal (“redstart”).

There is a secondary point regarding Cynthia: she may not count as a real moon, in spite of the fact that the word “moon” is now out there being used for ones which are. Isaac Asimov came up with the concept of the gravitational “tug of war”: the ratio of gravitational pull on a satellite between its planet and the Sun. He looked at the thirty-two known satellites in the Solar System at the time and found that of all of them, only Cynthia was pulled more by the Sun than Earth. He also found that the most distant moon of Jupiter know at the time, Sinope, was only slightly more attracted by Jupiter than the Sun. The Sun attracts Cynthia, however, more than twice as strongly as Earth does. Looking at the orbits of the planetary moons as they move around with their planets, you get a kind of “spirograph” pattern with them looping the loop. Cynthia alone doesn’t do this but is always concave to the Sun. It’s more like she’s just drifting along as our companion. Among the official planets, but not Pluto, Cynthia is also much larger relative to the size of her primary than any other body considered to be a moon. Hence the “Moon” is arguably not a moon at all but a companion planet. This, I admit, is a little like the botanical “nut” and “berry” situation, where bananas are officially berries but blackberries aren’t, and peanuts aren’t nuts but nutmegs are, but consider these sentences and which one sounds less peculiar: “The Moon is not a moon”, or “Cynthia is not a moon”. I would say the first sounds much sillier than the second. In fact I think we’d all agree that Cynthia is no moon, but we’d probably be thinking about someone we know called Cynthia who is not a ginormous ball of rock in space, which would be entirely sensible of us. For me, then, the word “moon” has a murky history where it was used to refer to said massive craggy sphere but that’s all in the past now apart from the few hundred million speakers of English who haven’t gotten with the program yet.

Then there’s the question of the definite article. We say “THE Earth”, “THE Sun” and “THE Moon” (well I don’t, but most people do), as if to pick them out and make them special. Now I do say “the Sun”. “The” is used a bit oddly in English compared to the use of definiteness in other languages which have that distinction. There are, for example, languages where omitting a definite article makes a noun indefinite, which doesn’t happen with us, and it often has other rôles common to many other languages which are absent in English where it tends to be more widely used, with proper nouns for example. “Earth” and “Sun” in these usages are indeed proper nouns, which don’t take the definite article in English. However, both words have other meanings: “earth” means “soil” for example, and “sun” refers to any star with planets. It’s fairly common for “Sol”, the Latin for “Sun”, to be used as a name for the Sun in the same way as Sirius A or Betelgeuse might be used as names for those stars, and again this has a Western bias which in fact is unusual for a star name, many of which are Arabic. The Arabic word for “The Sun” is “Al-Shams”, ignoring certain grammatical considerations. There are also Bayer designations to be taken into consideration, which are Greek letters followed by the genitive of the constellation the star is seen in from Earth. Clearly this can’t apply to the Sun here because it (“he”?) moves through the Zodiac once a year, but from α Centauri for example, the Sun is a bright star in the constellation of Cassiopeia and from τ Ceti, twelve light years away, it’s a rather fainter star in a constellation made up by Carl Sagan called the Six-Leggèd Unicorn (Monoceros Sextupedalis), at the base of whose tail we are situated. The constellation is unusually large compared to the ones in our sky.

Speaking of sky, this is also a bit of a planet-bound concept. It’s the view we have of the atmosphere and the rest of the Universe from our vantage point which is not blocked by the body we’re situated on. Space is not “up there” but all around us, and we are also in space. This is news to nobody of course, but it isn’t how we think of things in general. Wherever one happens to be within the atmosphere, the sky is above and Earth below. In order to be “in space” conceptually, we probably need Earth to occupy less than an eighth of our field of vision. The actual situation is complicated mathematically because it’s technically impossible to see an entire hemisphere regardless of one’s distance from a sphere, although one gets so close to being able to do so that this is rather fussy. The sky often refers to something which is almost an optical illusion where the rest of the Universe is obscured by the gas and clouds in the atmosphere, so it does exist during the day, but a clear sky at night is just a good view of part of our environment, to the naked eye up to about two million light years away but which we perceive as a black dome with pinpricks of light in it, plus Cynthia. Once again, we all know this. I’m aware I’m not saying anything new here, but although I reject the Saphir-Whorf hypothesis that our language completely determines our world, I do think it’s significant.

An illustration of how new this isn’t can be found in the work of the mid-twentieth century architect Buckminster Fuller. It was he who popularised the idea of “Spaceship Earth”, emphasising our interdependence on each other in a hostile void and the need to ensure that the systems which keep us safe here are maintained. Ironically, he was also a frequent flier. He used to speak of “Universe” as a proper noun without articles, which is of course similar to how I suggested dropping them for “The Earth”. The rationale behind this was “the aggregate of all humanity’s consciously apprehended and communicated (to self or others) Experiences”, a definition I feel is rather anthropocentric but which also acknowledges the fact that what we perceive just is the world to us. This brings to mind the error apparent in John Norman’s thought of confusing his own preferences with the wider idea of essential human nature, and as Norman has inadvertantly illustrated, the folly present in that confusion, which is something whereof we should all be aware. Buckminster Fuller’s frequent flying, environmentally unsound though it may have been, did also give him the insight of authentically experiencing Earth as a globe, and this influenced his use of the English language. For instance, he would talk of “world-around” rather than “worldwide”, in a move practically the opposite of the flat earthers in the recent satirical novel ‘The End Of The World Is Flat’, and it’s notable that this links to what might be seen as a more rational and just approach to humanity than “worldwide”, which suggests we’re not living on a globe. I personally find the specific phrase clumsy and would prefer to substitute “global” as more succinct and less intrusive, which makes it more likely to be accepted. He also substituted “in” and “out” for “down” and “up” respectively and used to talk about “going outstairs” instead of “upstairs”, emphasising the fact that we’re all clinging to the surface of a ball in space. That sounds precarious, but it’s worth considering our situation as precarious in a different way and therefore serves us as a reminder of that.

He also replaced “sunrise” and “sunset” with “sunsight” and “sunclipse”. The second sounds a bit artificial to me but the first is fairly okay, although still quite attention-grabbing in a way which doesn’t help unobstrusive adoption. Then again, calling Cynthia that doesn’t exactly seem unobtrusive either, so maybe I’m being hypocritical. In my unfinished novel ‘Unspeakable’, I refer to the limb of this planet concealing and revealing the Sun rather than sunset and sunrise, or something like that (I can’t remember the exact wording). Another approach is to refer to the terminator, which in astronomical terms is the locus of points on a body tangent to the Sun, enabling the synonymity of “my location crossed the terminator”, which can refer to either sunrise or sunset and emphasises movement and rotation rather than the illusory stasis we imagine we’re in.

Then there’s this:

The Australasian branch of the Society For Putting Things On Top Of Other Things is in a sense actually doing the opposite to what the Staffordshire branch didn’t do. Do they really deserve the praise of the chair? Although the angle isn’t perfect, what the Australasian branch have in fact done is put twenty-two things underneath other things. Alternatively, a less Eurocentric view would allow for the Staffordshire branch not to have done anything wrong and to have at least not undone the work of the Society. Then again, it appears that the Society as a whole does in fact grasp that Earth is round and gravity pulls towards the centre, and as a side issue the Society For Putting Things On Top Of Other Things does succeed in doing something very similar by putting things underneath other things, because the end result is that something is still on top of something else. What it’s actually doing, from a non-gravity dominated perspective, is putting things next to other things. If there is also a Society For Putting Things Underneath Other Things, they are not their enemies and in fact there could be a federation of societies for putting things next to each other to which they would both be entitled to belong. Their real enemies are such groups as the Society For Taking Things Off Other Things. Incidentally, a less well-known society is the Society For Putting The Letters “SPR” At The Spreginnings Of Sprertain Words, but their rôle is rather different, though also interestingly similar.

However, it is in fact important to know what’s on top of things on this planet, dominated as we are by gravity, and it would be dangerous to remove this distinction from language. It’s scant comfort to a crew trapped in a sub at the bottom of the ocean that they’re in another sense at the top of the ocean with a force pulling the water towards them, and their rescuers would be confused if they were to have the situation described to them as “the intermarine is situated next to a major phase change in matter” without specifying that that phase change was liquid to solid and therefore more likely to be at the bottom of the ocean than the surface. There’s a time and a place for these things and they aren’t always appropriate. Nonetheless, our intuitions can be misled by using language based on outmoded concepts such as these, which are particularly outdated for two reasons: they are based on a flat Earth, which was superceded in Ancient Greek times, and also a geocentric view, which began to be replaced five centuries ago.

Another aspect of this is the realisation that spacetime is a single set of relationships rather than two separate things, meaning that, for example, a unit such as a light year is a measure of spacetime and not just distance as we’d usually understand it. Relative to us, light travels in a diagonal line, and its spacetime coördinates are four-dimensional, as is everything else. Hence when we consider Earth’s rotation and her orbit about the Sun, among other forms of motion, we are in a sense referring to angular motion when we use ideas about the passage of time to some extent. At midday any location on the Equator is 90°from the terminator in all directions across the surface of the globe. Although the situation is harder to describe in different places on Earth, the fact is that time of day can still be considered to be an angular measurement in our planet’s rotation. Likewise with the year, which is close to amounting to a degree’s movement per day although it’s slightly under on average and Earth also accelerates and decelerates somewhat according to time of year, being fastest near the northern summer solstice and slowest half a year later. Of course, the whole Solar System is orbiting the black hole at the centre of the Milky Way once every 225 million Earth years or so, meaning that Earth is describing a shape locally similar to a helix but in fact part of a larger approximate helix. Moreover, the Great Attractor in the direction of Virgo is pulling the Local Group of galaxies, including our own, towards it, and space itself is expanding, although that has little bearing on most of the rest. It might mean that whatever is pulling us all towards Virgo will be more distant in the sense that it will take longer than might be expected at a constant velocity because it will in a sense be in a different place.

There’s also the question of the light cone. This is in fact a sphere of influence rather than a cone, concerning the distance between points which can influence each other in a given time. Say a star explodes. After ten years, the explosion will be visible ten light years away, after a hundred, it will be visible a hundred light years away and so on. Its sphere of influence spreads out at a maximum speed equivalent to light’s. Therefore it may not make much sense to consider that anything really occurs simultaneously. If something is happening now ten light years away, it’s impossible for it to make any difference here for at least a decade. For this reason, again in ‘Unspeakable’, I used a calendar system based on the Crab Nebula pulsar about five thousand light years away, with the date beginning at the instant light reached the location in question, and with units of time based on the period of the pulsar, which is very gradually slowing. Hence because the Crab Nebula was first observed on Earth in the year 1054 CE, I chose that as the year zero for us, but for Antares that calendar would begin in about 1600 CE because it’s more than five hundred light years further away from that supernova. I was trying to illustrate the ties between time, space and causality by doing this, and in fact I’m quite keen on the idea that such a calendar would work for real. In practical terms it would make very little difference on this planet because it only takes light forty-two milliseconds to cross Earth’s equatorial diameter, but using the period of the pulsar as a unit of time takes it away from Earth- or solar-based units. The current period of the Crab Nebula pulsar is approximately 33.1 milliseconds, a figure insufficiently accurate to base a calendar or clock system on. SN1054 took place on 4th July 1054, which was Julian Date 2106209. Today’s date as I write this is 18th September 2021, or Julian Date 2459476.08125 (it’s 1:57 pm). The tropical year 2000 was 365.24219 days long, which is 31 556 925.22 seconds. However, it makes more sense to treat this in terms of days rather than years, which makes it 353 267 days since we saw SN1054, or 30 523 046 400 seconds, bearing in mind that the exact time of night was not known. In terms of current pulsations, which will have slowed a bit by now, that makes 922 146 416 918.429 with spurious accuracy. I have to say that using base ten to express this is not ideal, and in the case of timekeeping, we are in fact used to not using that radix anyway, as is the case with angles.

A little while ago, I wrote a post considering what Latin would be like today if Rome hadn’t fallen, bearing in mind that Latin does survive as an everyday widely-spoken language in the form of languages such as French, Romanian and Catalan. In particular, something to consider here is that scientific nomenclature would probably have arisen directly from spoken language rather than having been mainly based on Latin and Greek but without native sensibilities or a firm grasp of the language itself. Hence elements could be referred to by their atomic numbers directly, which does happen today for placeholder names to some extent, as in “ununpentium”, now known as moscovium but clearly dependent on Western Arabic numerals used in decimal and employing place value. Similarly, when Uranus, Neptune and Pluto were discovered, they were given classical names in accordance with the spirit of the names of the other planets but perhaps not in direct accordance with how “modern” Romans would have named them. Hence it’s easy to imagine a language which is somewhat like Italian and Romanian but uses different, though still classically-based, technical terms. It’s also possible to decouple these terms from the vagaries of history and the techology available when they were first discovered, leaving us with a more logical scientific vocabulary. There are in reality tendencies to address this in human anatomy, where we no longer speak of Fallopian tubes and the Achilles tendon but uterine tubes and the calcaneal ligament. It would be interesting to address this across the board and see how it changed our way of thinking, but it’s also difficult to anchor it accurately because new discoveries are being made all the time which could turn this upside down. Whatever we came up with would become a kludge in the long term and need a rethink.

To conclude, we are imprisoned on this planet and in our present state by the way we use language. It’s very uncomfortable and interferes with communication and clarity to mess about with it too much, but it’s also profitable at changing how we perceive the world, and might enable us to come up with new outlooks and solutions in the long run. Hence although all this is a game, it’s quite a serious game and it’s worth playing if we achieve some kind of conceptual breakthrough as a result.

The Ring Earth

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Not to be confused with Ringworld or yesterday’s post, this is about whether a doughnut-shaped planet could exist, but just to clear that up, Ringworld was a concept thought up by Larry Niven for his ‘Known Space’ series of a megastructure consisting of a ring-shaped terrain orbiting a star and given day and night by rectangular shades orbiting further in. It would require as yet undiscovered materials, in other words unobtainium, not to be confused with unobtainium, to be built, although a more diffuse ring of habitats or indeed planets in a single orbit is entirely feasible. This is not that.

Relatively small bodies in this Solar System are representative of the possible shapes which can be achieved by given amounts or quantities of solids, liquids, gases and composite matter made thereof. The fourth state of matter is entirely different but planets are not made of plasma, practically by definition. The smallest approximately object gravitationally obliged to be round is of course the Death Star moon Mimas:

Even this isn’t that close to being perfectly round because of the relatively huge crater Herschel, but it can also be seen that it has a noticeably rough outline in this picture. It’s allowed to have fairly large mountains and deep craters. Mimas is around four hundred kilometres in diameter, although it deviates by about twenty from this, but that’s still pretty round when considering that the deepest trench in our ocean is almost twenty kilometres lower than the highest of our mountains, compared to sea level which is not perfectly spherical itself. Mimas is in absolute terms as close to being round as Earth is, given that our mountains, valleys, trenches, continents and abyssal plains were not scaled down on a Mimas-sized model of our planet.

The next Saturnian moon down from that size appears to be Hyperion:

So far as I can tell, just as Mimas is the smallest roughly round world in our Solar System, so is Hyperion the largest object which is a long way from being round at 360 x 266 x 205 kilometres. It’s within about thirty kilometres of Mimas’s smallest diameter, yet it manages to be rather irregular. It looks more like a pebble of pumice than a planet, and of course it’s neither. Its mean density is only just over half that of water, which is actually lower than pumice, and also lower than that of Mimas, which is about 1.15 times water’s. There must be a complicated relationship between strength, rigidity and density which decides the shape of objects of about Hyperion’s and Mimas’s size.

This house has many Escher prints owing to my family’s joint enthusiasm for the artist. One of my favourites, which I’m sad to say is not on any of our walls, is ‘Double Planetoid’. This shows two intersecting tetrahedra, one completely unaltered by technology, the other completely covered in building. Of this, Escher says:

‘Two regular tetrahedrons that penetrate one another, float through space like a planetoid. The light-coloured one is inhabited by human beings who have completely transformed their region into a complex of houses, trees and roads. The darker tetrahedron has, of course, remained in its natural state, with rocks on which plants and prehistoric animals are living. The two bodies fit together to make a whole but they have no knowledge of each other.’

M C Escher

It would be possible to make a real copy of ‘Double Planetoid’ somewhere in space, at its approximate scale. In fact it would also be possible to scale it up to at least twenty-seven kilometres on a side if it were carved from granite, even if it had Earth’s gravity. However, it probably wouldn’t arise without intelligent manipulation being involved somewhere, except maybe in an infinite Universe or a parallel world somewhere. It would also not generally be possible for ordinary matter to exert sufficient gravity to make a real version of this rather than a model.

I bring this up because clearly an object like Hyperion could be sculpted into a particular shape, although in its case this would probably already be constrained by gravity so it might end up quite rounded off, but one like Mimas couldn’t. Again, there are likely forms something could take other than round, usually just irregular and lumpy, when they’re fairly small, and many of these are seen in asteroids and small moons. 433 Eros, for example, is often described as “sausage-shaped”:

(I’m not sure that’s how I’d describe that). The asteroid Cleopatra is one of several described as dumbbell-shaped:

Once an object is the size of a planet, though, the options for possible shapes closes down a lot. Considering this as an Earthling, the tallest possible cylindrical column of granite is said to be about the height of the Matterhorn, around half that of Everest at 4 478 metres, and the tallest possible pyramid of granite is 13 400 metres, on this planet. However, this can’t be strictly true because if the height of Everest and the depth of the Marianas Trench are added together the total comes to 19 882 metres, so given a wide enough base this can be exceeded. The diameter of the geoid – the shape of this planet defined by the level water would reach given only the influence of gravity and rotation on this planet, which approximately means sea level – varies by over thirty kilometres between the poles and the equator, which is again somewhat more than twice 13.4 kilometres. Twice is fine because we’re talking diameter rather than radius, but more than twice suggests there are other influences, such as rotation. Steel can, if I recall correctly, form a cylindrical column up to thirty kilometres high and there are a few specialised substances which could be used to build a tower which officially reaches into space, but they’re exotic and would have to be specially synthesised.

This full-disc image of Jupiter was taken on 21 April 2014 with Hubble’s Wide Field Camera 3 (WFC3).

The planet with the most obvious deviation from spherical in this Solar System is Jupiter, which has a polar diameter of 133 708 kilometres but an equatorial one of 139 820, which is a variation of 4.5%. This is because Jupiter is a substantially fluid body consisting of liquids and gases, and because it spins very fast. In terms of velocity the planet’s equator is moving over thirty times faster than Earth’s and it’s also over three hundred times our mass. However, Jupiter happens not to be the least spherical planet in our neighbourhood. That honour goes to Saturn, whose rings may disguise the fact. Saturn has an equatorial diameter of 116 460 kilometres and a polar one of 108 728, which is a variation of over seven percent. This may be connected to the fact that it’s also the least dense planet.

It’s established, then, that a planet can be tangerine-shaped rather than spherical if it’s sufficiently fluid. These two examples are also large and rotate fast. Earth is not like that, but it’s theorised that there are planets out there in the Universe which are mainly made of water, or which have extremely deep oceans. These could presumably assume such a shape, which on a planet the size of Earth would be like having a stationary wave almost a thousand kilometres high circling the equator, and in fact this would even be noticeable from the surface as it’s close to a gradient of one in ten. This could be described as a tangerine-shaped planet, but I have to say I don’t find that idea very interesting. The shape is officially called an “oblate spheroid”. There are stars which are markèdly flattened in this way, such as VTFS 102 in the Large Magellanic Cloud’s Tarantula Nebula, which is three times wider at the equator than at the poles, but stars are not made of solids, liquids or gases.

Another well-known variation of a spheroid is the rugby-ball shape, or prolate spheroid, and there are also stars of this shape. Some binary stars orbit each other so closely that they are mutually distorted into elongated shapes of this kind, and I don’t know this but it seems possible to me that this is also because of how fast they’re whizzing round. The question arises of whether a planet could have such a shape. Larry Niven, again, imagined such a planet in the Sirius system he called Jinx, whose “poles”, so to speak, were effectively vast plateaux rising out of the atmosphere at each end, and the “equator” was a high gravity area. The humans living near the equator needed to be very strong and muscular to cope. I don’t feel convinced that this is possible for a largely solid planet, but just as Saturn and Jupiter can get squished by their rotation I can see that if there is a system somewhere with a double gas giant, this might be what shape those planets would assume. The same might even apply to double deep ocean planets.

Other possibilities are very limited. For instance, an egg-shaped planet flat at one end and pointed at the other is difficult to envisage. However, there is one possibility which, oddly, is very far from being spherical but is still possible.

I’ve mentioned the periodical ‘Manifold’ on here a couple of times. This was a mathematical magazine published by the University of Warwick, one of my almæ matres, from 1968 to 1980, one of whose claims to fame is that it invented the game ‘Mornington Crescent’. I used to read it back then, and one of its many whimsies was a fictional toroidal planet whose name escapes me, with six cities all joined together by an underground railway. This is a reference to a well-known mathematical puzzle involving three houses all of which need a water, gas and electricity supply but none of the pipes could cross each other. This is impossible to arrange on a flat surface but works fine on a torus.

When I read this, I mused that it was a shame that such a planet could never exist, and I started working out things like I did above, with the likes of pyramids as very high mountains and various irregularities in its surface ruling it out. I then realised that I couldn’t actually find a reason for such a planet not to exist, and just assumed I didn’t have the mathematical prowess to work out why it couldn’t.

Well, it turns out that it can, in the sense that if a toroidal object of Earth’s volume, mass and composition could be formed in the first place, it wouldn’t be susceptible to collapsing into a spheroidal form. The above shape, surprisingly, is gravitationally stable. Incidentally this would also apply to water in free fall, so a spinning doughnut shaped swimming pool in space made entirely of water is completely feasible, under a pressurised atmosphere of course. There’s a fairly easy way of understanding this. It’s already been shown that tangerine-shaped planets exist, which are largely fluid and flattened by spinning. This is a kind of limiting case of that situation. If a fluid planet ended up spinning fast enough, not only would it become flattened but its matter would be completely pulled away from its axis of rotation. Most planets can be considered either to start out as fluid, i.e. they are either actually liquid, such as made of magma, or of sufficiently small lumps of matter that they behave on a planetary scale as if they were, just as an actual liquid consists of molecules or a heap of sand or dust can flow like water, have surface waves and even drown people. The difficulty is in imagining a scenario where this would actually happen on its own. The alternative is simply to say it’s being done by intelligent life but then the imagination falters a little as well, because how powerful would a civilisation have to be to have the resources to make its own planets? Also, why?

Nonetheless, however it came into being, once it was there it could continue as long as any other planet in its current shape, and this is a little surprising because the deviation from a sphere in this case is extreme. I also have to admit to a little confusion and have to insert an explanatory note. I can’t honestly tell whether this shape is sustainable simply due to the planet’s gravity or whether it would also need to be rotating fast with the axis passing “vertically” through the hole. I suspect the former is the case, but even if it isn’t, the second case would guarantee that it’s possible, although it’s not clear how fast it would have to be spinning. That would also depend on the proportions of the torus. Now for the explanatory note. Thus far I’ve assiduously avoided using the words “centrifugal force” because that doesn’t exist as such, as is well-known, but it can be quite awkward to express oneself without using those words. What is in fact happening in this situation is that the mass of the planet is constantly “trying” to move in an infinite number of straight lines, all tangent to its surface at the outer equator, but is pulled away from that path owing to the electromagnetic and gravitational forces holding it together.

It’s also very unclear how big this planet would be. According to the second picture at the top of this post, the north-south distance across Afrika is rougly equivalent to the width of the “tube” of the torus, the same distance for Australia is its thickness and the hole is about the same size as Australia again. Hence that version of a toroidal Earth is 3 000 kilometres thick, 7 000 kilometres wide on either side and has a hole 3 000 kilometres in diameter. This raises two questions for me about how to calculate the volume and surface area of a torus and also what to call the different features of the shape. Strictly speaking, the shape in the picture is not a torus because it’s not circular but elliptical in cross-section. The distance from the centre of the hole to the outer edge is called the “major radius”, R, and that from the centre to the inner edge is the “minor radius”, r. There’s also the aspect ratio, which is R/r. Strictly speaking, not only is the above not a torus (although the blue image is), but even if it was, it would only be a particular kind, namely the ring torus. There are also horn and spindle tori. A horn torus has its circular cross-sections touch at the centre, so strictly speaking has no hole, and a spindle torus has the circles overlapping. Both of these shapes are slightly more achievable for a planet than the ring in terms of events happening without intelligent intervention.

The formulæ for surface area and volume are respectively:

where p=R and q=r. This suggests several “equivalences”. One is the size of a torus with the same surface area as Earth’s, another the volume of such a torus, another the size of a torus with the same volume as Earth’s and another the surface area of that torus. All of these are also dependent on R and r, and thus the aspect ratio. I’m not going to address these immediately.

The torus in the second picture has basically the same continents as the real world, but Antarctica seems to be missing. In fact it can be concluded that the polar regions are missing altogether. However, there are two circles corresponding to the poles and of course two further circles corresponding to the Equator. Assuming the planet is held together by its rotation and doesn’t have constant daylight anywhere on its surface, i.e. not rotating with the hole axis facing the Sun, the “polar” regions are pretty close to lands which are equatorial on the real Earth in that image, although the other side has another circle which in the Arctic. Meanwhile there are inner and outer equators, and the outer passes through the Mediterranean. Assuming no axial tilt the inner equator is in eternal darkness and therefore colder than Antarctica, which would take a lot of water out of circulation and probably cool the whole planet. If it’s tilted at the same angle as we are, on the other hand, it would be exposed to sunlight some of the time and the “polar” circles would also have seasons, half a year of night and half of day and so forth, as they have here.

If this planet maintains its shape through rotation, there will probably be strong winds and ocean currents everywhere. There’s also an important topological difference between a spheroidal and a toroidal Earth. Topologically, considering the troposphere (the bit with the weather in it) as a single layer, there must always be at least two locations on Earth where there is no wind. This is not so on a toroidal planet because the hypothetical still spots could be lined up to be in the hole. Ocean currents are like this in the real world, because the land punches holes in the ocean in which potential still points could be located. If you go high enough in Earth’s atmosphere, the air is no longer dragged along by our rotation, so perhaps a toroidal Earth could have a relatively calm troposphere like ours is.

Apparent gravity would also vary across its surface. The rapid spin would act against gravity at the outer equator and in favour of it at the inner one. Some time ago, Alfred Wegener attributed continental drift to the centrifugal effect he called Pohlflucht. Arguably, as it depends on how rigid the planet is, Pohlflucht could be a reality on this world. Perhaps the continents actually would cluster around the outer equator. If they did, though, they would have to be quite mountainous to prevail over the water, which would be pulled into a belt in the same region. This, however, might actually be so because the lower gravity would favour higher mountains in that area. It seems to be shaping up into a situation where the inner region is a cold, flat desert, there are two strips of land either side of the outer equator along with a tendency for continents to move towards the outer equator where they form fold mountains which are, however, submerged under a deep ocean, which resembles the Tethys of our prehistoric past.

There needs to be a moon of some kind to generate a protective magnetic field. This could orbit at the outer equator. The toroidal magnetosphere thus formed would be a different shape than the real one.

From the surface, there are conventional horizons to the north and south, but to the east and west the vista depends on where you are. On the outer equator, the situation is pretty much as it is here. Near the “polar” circles, the planet is effectively flat along one circumference, and the landscape or seascape (snowscape more likely) disappears into the haze of the atmosphere. The inner equator offers the most spectacular view. During the day, the sides climb upwards into curved, hornlike shapes which gradually plunge into night, forming an overhead arc. At night, the situation is the same but there would be a visible daylit sector which would first recede up the horn, travel across the sky and then descend towards the observer until dawn. On this inner surface, gravity would be high, so the view might be nice but it would also be quite uncomfortable or even uninhabitable. I’m assuming here that there would be an axial tilt.

There’s a limit to the relative size of the hole. The narrower the ring, the less stable the planet. Both of the first two illustrations are viable, but a more traditional banded ring shape would be highly volcanic because it would tend to flex and crack under the forces maintaining its shape. Hence a doughnut shape is best. Even then the day would only last about three hours. A moon might move in a straight line in and out of the hole, or it could follow an ∞-shaped orbit.

The remarkable thing about this scenario is, of course, that it isn’t impossible. There could never be a tetrahedral or cube-shaped planet and the largest conceivable regular polyhedral planet would probably be something like a dodecahedron perhaps somewhat larger than Mimas but still much smaller than Earth or even Mercury, because the vertices would effectively be high mountains. In fact planets are in a sense polyhedral because they aren’t perfectly smooth spheroids on one scale, although on a smaller scale the jagged peaks and steep valleys would be rounded – this is a fractal issue, because on a smaller scale still they’d be jagged again, and so on. However, they are also very close to being smooth. As far as I can tell, the only possible shape a planet could be which is radically different from a sphere is a torus. What isn’t clear is whether it could ever happen on its own. I can easily believe that there are occasionally asteroids which have holes all the way through the middle, although as far as I know there are no known examples in this Solar System. A very rapidly spinning protoplanet could form into a torus, and the question then arises of what could cause it to spin so rapidly. Perhaps if it were high in iron and close to a neutron star this could happen, but it would be unlikely to be habitable. A non-habitable toroidal planet is unsurprisingly much easier to devise than a habitable one. However, given the will, the technology and the access to resources, nothing at all seems to stop an intelligent technological culture from making such a planet on a whim, or perhaps as a work of art. Isn’t that amazing?