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.

Triton

Triton, along with the similarly-named Titan and also Ganymede, is one of the largest moons of the outer system. Before Voyager 2 reached it, it was considered possibly the largest moon of all. Moreover, apart from our own highly anomalous Cynthia, it’s large in proportion to its planet’s size. Using the largest moons of each planet, the proportions of their masses work out thus:

Earth:Cynthia – 81
Jupiter:Ganymede – 12 808
Saturn:Titan – 4 222
Hamlet:Titania – 25 294
Neptune:Triton – 4 768

In terms of equatorial diameter, the ratios work out thus:

Earth:Cynthia – 3.67
Jupiter:Ganymede – 26.54
Saturn:Titan – 22.62
Hamlet:Titania – 32.15
Neptune:Triton – 18.19

Just for reference, the ratios for Pluto:Charon are 1.96 for diameter and 8.22 for mass, but Pluto‘s status as a planet is not unquestioned. It can in any case be seen that of all the large moons, Neptune’s Triton is still in proportion and there’s a big gap before our own special case, but it is still unusually big.

A common mechanism for the formation of moons is for the region around a planet to behave like the solar nebula did when the planets themselves were formed, with eddies in the cloud pulling in matter as the planet takes shape. Hamlet’s moons may be an exception to this, as they may result from the trauma that planet underwent. Outer and irregular moons are, however, often the result of captures and this is particularly evident when they orbit the opposite way from most Solar System bodies, and Triton is by far the largest body to do this. This has been known since its orbit was plotted in the nineteenth century. Due to its size and therefore relative brightness, the moon also holds the record for the shortest gap between the discovery of its planet and its own, as it was found in October 1846 CE, only a month after Neptune. This, however, is not as impressive as it sounds because all the planets out to Saturn have been known since ancient times and Pluto is very small and may not be counted as a planet, so it basically means that of the two planets discovered in the telescopic age, one of them has a very large and relatively bright moon which was easy to spot.

Certainly by the ’70s, Triton was, as it still is, considered to be a captured planet, though that would probably generally be qualified as “dwarf” now. Given the controversy of what counts as a planet, Triton of all worlds in the system has surely got to be the closest to that definition, as although it may have undergone the mishap, if that’s an appropriate word, of being grabbed by Neptune, it’s quite large and massive and probably used to dominate its orbit, as the 2006 IAU definition demands. Strictly speaking, and perhaps by being a bit arsy, Earth doesn’t even count as a planet by that definition. Hear ye then: Triton is a planet. I was first introduced to this piece of information in the ’70s, which is how I can make that provisional estimate of its timing, and consequently looked forward to the Voyager missions as including an encounter with a body likely to be very like Pluto. At the time there was little prospect of a mission to that planet, so it was the best I felt I could hope for.

The Voyagers took advantage of a rare planetary alignment which only occurs once every two centuries and started in 1976, dubbed the “Grand Tour”, which would allow probes to visit several planets in a row. This idea dates from at the latest 1971, and there were initially three possibilities. Two involved Jupiter, Saturn and Pluto and the other all the gas giants, but it was impossible to visit both them and Pluto on the same mission, at least efficiently enough to be practical. The ultimate decision was to take the last option, although Voyager 1 is a bit like the first two with the omission of Pluto. Also, although the Voyager 2 mission resembles the final option quite closely, it isn’t actually the same as the initial plan, which involved launching in 1979, visiting Jupiter and Saturn in ’81 and ’82 respectively, Hamlet in ’86 and Neptune in ’88, as the Voyagers were launched in ’77. The earliest option for Pluto inolved a ’76 launch, visits to the two inner gas giants in ’78 and ’79 respectively and Pluto in ’85. Although the final choice was, I think, a good one, it’s interesting to contemplate what might have been. It would be disappointing not to have visited the ice giants but amazing to have got to Pluto so early, and it also seems very likely that if that had happened, Pluto would never have been demoted. However, it was not to be, and this makes Triton a kind of Pluto substitute. It is in fact very likely to be similar to Pluto and it’s worth comparing the two.

Excluding the Sun, Triton is the fifteenth largest body in the system, Pluto the sixteenth. Eris is next on the list, incidentally. In terms of mass, Eris is between Pluto and the more massive Triton. Circling Neptune, Triton takes 165 years to orbit the Sun , Pluto 248, which is close to a 3:2 ratio (lots of ratios in this post for some reason) like the other plutinos. Considering its similarity, it seems likely that Triton was itself a plutino with a 248-year period like Pluto’s (which is what defines them), and right now I’m also wondering whether some of the other moons of Neptune, particularly Nereid with its peculiar orbit, were in fact originally moons of Triton. I expect this has already been researched.

Being retrograde is not the only peculiar feature of Triton’s orbit. It also varies its tilt through a cycle corresponding to only four Neptunian years, and is moreover remarkably round, by contrast with Nereid’s. Its distance from the barycentre (centre of gravity between two bodies) varies by less than six kilometres each way. This may be the roundest orbit in the Solar System and is quite remarkable. Our own orbital eccentricity is a thousand times greater. Hence there are a few combined mysteries here, which are probably related: the moon orbits backwards, shifts rapidly (over a period of about five centuries) in how tilted its orbit is and hardly varies at all from its mean distance of 354 759 kilometres from the barycentre, which is around seventy-five kilometres from the centre of Neptune. The size of the orbit is also only a little less than Cynthia’s around Earth. I have an illustration by Luděk Pešek of the moon in Neptune’s sky, painted in the early ’70s, and at the time it was considered much larger than Cynthia. It’s now been found to be somewhat smaller at 2706 kilometres diameter, and is of course somewhat less dense due to its ice content, although Cynthia, being formed from the Earth’s outer and lighter layers, is only about 50% denser. That said, Triton still averages over twice the density of water, making it one of the densest objects in the system beyond the orbit of Jupiter, and also denser than Pluto. Given the nature of its surface, this is all the more remarkable, and I’ll come to that.

Before its capture, Triton would’ve dominated its region of the system beyond Neptune, and perhaps even have counted as a planet in its own right by the IAU 2006 definition. Neptune is in a peculiar position regarding the Bone-Titius Series, and if that is in fact a law of nature it could be expected to have been somewhere else in the past. This would presumably in turn have meant that the plutinos have fallen into orbital resonance with it since it moved and the presence of small, solid planets beyond its orbit would lend the Solar System a pleasing symmetry, with small rocky planets in the inner system, gas giants in the middle and a further succession of small icy planets beyond them. It is of course highly speculative to suggest that Neptune used to be somewhere else. Olaf Stapledon supposed Neptune to be followed by a further three planets, of which Pluto was extremely dense and made of iron, because only with such a hefty planet would be able to perturb Neptune to the extent it is. It was common at the time for scientists to presume this as they’d predicted Pluto’s existence from these perturbations, but I’ve gone on about this elsewhere.

Pluto and Triton are almost the same in composition, suggesting a common origin. The moon’s surface, however, is somewhat different. It’s unusually flat, with variations in elevation of less than a kilometre. It also has a surprising composition: it’s made of frozen nitrogen. At this distance from the Sun, the gas which makes up most of our atmosphere composes the solid, though also soft, surface of a world. It’s therefore no surprise that the surface temperature is exceedingly low at -235°C. However, there is also a greenhouse effect, in this case considerably more literal than usual. The nitrogen forms a clear surface which traps the sunlight just below it, heating the subterranean nitrogen and causing it to erupt out of the surface like geysers or volcanoes to a height of around eight kilometres. This then drifts downwind by as much as a hundred kilometres, leaving streaks on the landscape. This process also maintains the moon’s nitrogen atmosphere, which is thin by terrestrial standards but not as tenuous as many of the atmospheres of other moons, at fourteen microbars. Although this may not sound like much, it’s enough to be a collisional atmosphere. That is, the molecules in Triton’s atmosphere are near enough to one another to come in contact at least occasionally, which means the air behaves as a fluid like air at sea level on Earth, rather than just bouncing around or orbiting the moon as it does on our own. Even so, Triton’s atmosphere is a lot thinner than expected. The lower the temperature, the easier it is for a body to hold on to gases and perhaps liquids if the atmospheric pressure supports them. Nonetheless, Triton doesn’t seem to be very good at it. Its surface gravity is 0.0794 that of ours, over half that of Titan, whose atmosphere is several times denser than Earth’s and whose temperature is something like two and a half times higher. There’s a small amount of methane in the atmosphere too, making it like a much thinner version of Titan’s, but also colder since it’s below both substance’s freezing points. Just as an aside, it’s been conjectured that of all the substances likely to form oceans on planets or moons somewhat similar to Earth, i.e. oceans on the surface along with land masses or islands, nitrogen would actually be the most common liquid of all, with water only coming in second. Triton is not a world with permanent bodies of liquid on its surface, but like Cynthia, it does have large flat plains of solidified “lava”, in this case frozen nitrogen, which contributes to its general flatness. Unlike water, most liquids freeze “under” rather than “over”, so the frozen nitrogen lava plains of Triton would have done so by cooling on the surface and then precipitating down inside the body of liquid, gradually filling up until the whole lake or sea was frozen solid, except that it would then have melted and vaporised in some places and pushed through once again. The geysers are near the south pole, similar to the Enceladus situation, but this is a much larger and heavier world than that moon. However, there are also claims that the lava is in fact an ammonia-water mixture, so all of this is provisional. The fact remains that most of the atmosphere is nitrogen.

The resolution of the picture at the top of this post is surprisingly large considering it’s a mosaic of images captured by a camera from the mid-’70s. Although it’s diminutive on this page,clicking on it will show it in its full glory. Pixels are only five hundred metres across at the centre, so this is a pretty detailed map of most of the surface and would show medium-sized parks if it were a picture of Earth. It’s like a photo of Earth from the ISS, although of course the whole of our planet wouldn’t be visible from such a distance. A distinctive feature is the so-called “canteloupe terrain” because it looks a bit like this kind of melon:

Triton’s version looks like this:

The winding heights are several hundred metres high and a few hundred kilometres across, and the plains they surround are safely two hundred kilometres wide, which is significant for a moon which, though large, is only about ten times that in diameter. The ridges consist of water ice which has been squeezed upward, and the whole surface of the moon is quite young as it has few craters. It could even be Cenozoic. This is possibly a surface which didn’t exist when T. rex walked the earth, although another surface did. To my mind, this raises the question of whether Triton was actually an independent planet at the time and if this melting can be blamed on the capture.

The similarity of the smooth basins to lunar maria will not have escaped you. The difference is that whereas those are made of basalt, these are nitrogen, as I’ve said. It’s worth bringing up again though, because on different worlds at different temperatures the same kinds of processes and structures exist but are made of different substances. On the whole, most substances which can be solid, liquid or gaseous in a given situation without major changes are, unsurprisingly, broadly subject to the same kinds of physical laws. The exception, more surprisingly, is water, because in the state with which we’re familiar, that is, under enough pressure to give it a liquid phase but only enough to ensure it has the most loosely spaced solid one, it expands and therefore floats when it freezes. This would have consequences such as the canteloupe terrain on Triton, which could be caused by its expansion as it solidified. Ironically, liquid nitrogen and molten rock (a bit of a generalisation) have things in common which they don’t share with water, a highly anomalous substance, due to water’s expansion on cooling and surface tension, among other things.

The solid nitrogen on Triton can be seen as the slightly blue-green streak across the image at the top of this post. It’s actually β nitrogen, which forms hexagonal crystals although they don’t form arrays like graphite or honeycomb. I can’t swear to this, but since the element immediately below nitrogen in the periodic table is phosphorus, whose least derived form is the dangerous but waxy white phosphorus, and I suspect that solid nitrogen fairly close to its triple (“melting”) point is also like this. This is not a thorough scientific appraisal so much as a hunch. White phosphorus slowly combines with oxygen in Earth’s atmosphere, and nitrogen as such is highly reactive, hence its use in explosives, but generally reacts with itself to form a highly inert gas at temperatures compatible with human life. On Triton, whether or not it’s reactive it may not have much to react with and the lower temperature would inhibit many such reactions. The issue here is really that although, as I’ve said, in some circumstances it hardly matters whether the substance in question is silica or nitrogen, as both can form volcanoes, erupt, produce lava flows and the like, such a substance as solid nitrogen or a mass of liquid methane on a lake on Titan is far from our own experience and our expectations can be misleading. However, it does seem highly feasible that the plains of the canteloupe terrain and the general flatness of the landscape is due to the waxy softness of the nitrogen which forms part of them. At this temperature also, water ice is almost a normal solid, expanding with increasing temperature and contracting as it cools, but it has clearly passed through the anomalous phase we think of as normal behaviour for a liquid.

What’s Triton’s interior like? Nitrogen in this solid form is very slightly denser than water at our freezing point, so it unsurprisingly covers the surface and forms a substantial part of the crust. The moon is rockier than the other moons trans the asteroid belt with the exception of Io and Europa, which are basically just balls of rock like the inner planets with a thin coating of other substances. Triton does still have an icy mantle but it will have a rocky core high in metals like a terrestrial planet’s. The brightness of the nitrogen surface cools the moon while simultaneously heating the upper layers of the crust, making it one of the coldest worlds in the known Solar System. The geysers are driven by the heat of the Sun, such as it is, emphasising what looks to us from here, close in to the Sun, to be a thermally delicate state. It might be expected not to last long in its present form when the Sun becomes a red giant, but the same is true of Earth. Solid and liquid matter as such is not the kind of thing which can cope well with the kind of temperatures found near stars. There’s also the “logarithmic” effect of low temperatures. The freezing point of water is about half the temperature of a hot oven and its boiling point at sea level is less than twice the temperature at our South Pole in midwinter. Nitrogen and oxygen have similar melting and boiling points at the rather mind-boggling sea level atmospheric pressure, and to us the fourteen degrees of difference between the boiling and freezing points of nitrogen sounds very narrow, but if centigrade had been standardised with nitrogen instead of water, absolute zero would be -550 degrees below zero. There’s an effectively infinite range of temperature before reaching absolute zero, which is like the speed of light in that respect – effectively inaccessible and some kind of ultimate limit.

Although they have their own smaller moons, Pluto and Charon are effectively a double planet system. It’s been theorised that the same was also true of Triton before its capture. Many other Kuiper belt objects are binary, and modelling of the dynamics of capture show that Triton is more likely to survive if this was so. The other object would be ejected from the system. To my mind, this contrasts with Hamlet’s situation, where a similar collision may have resulted in the “moon”, such as it was, being incorporated with the substance of the planet itself and also disrupting its axial tilt. The question then arises of where Triton’s companion might be now if it survived the encounter, and in my current ignorance I wonder about the similarly-sized Eris.

The name Triton originates from Poseidon’s (i.e. Neptune the god’s Greek counterpart) son, and has been more widely used for other purposes than most other names of major planets and moons. For instance, this is a triton:

This is the animal that first springs to mind for me when I think of newts, but they are nonetheless known as tritons. It’s also used as the name of a sea snail and a species of cockatoo. The list is much longer than for many or most other names also used for celestial bodies, which seems rather anomalous to me and possibly reflects the relative obscurity of the moon compared to some others, though maybe I’m out of touch in saying that.

Neptune’s satellite system as a whole is sparser than the other gas giants’, with only fourteen known moons. Until the ’80s, only two were known. This may be connected to Triton’s presence, either enabling it to remain without disturbance or maybe due to its own disturbance of the system. When Triton first arrived, its surface is likely to have been molten for an æon. In Triton’s case this presumably means a liquid nitrogen ocean over a water ice bed, which makes it seem that it was captured in the late cryptozoic eon, if that estimate is at all accurate. Hence over the period when Earth was almost frozen over itself and had little or no surface liquid, Titan and Triton both had oceans, and the latter would’ve been a possible member of the very large number of worlds with liquid nitrogen bodies of liquid on their surfaces, which is plausible but unknown. It’s also unclear whether it had landmasses. But in any case, the number of moons is surprisingly small. The comparably-sized Hamlet has more than two dozen, but Neptune only has fourteen. All but two of these were unknown before Voyager. Triton’s mass is two hundred times the mass of all the other moons put together.

As a world, Triton is somewhat smaller than Cynthia. Its surface area is 23 million square kilometres, 40% of which has been imaged. This makes it bigger than any country and a little larger than North America, but smaller than Afrika or Eurasia. It seems entirely feasible, probable in fact, that its surface is covered by more nitrogen than is present in our own atmosphere. Triton and Pluto both have irregular pits with cliff edges on their surfaces which are not craters, called “cavi”. Ten of these have been named, all after water spirits. Cavi usually occur in groups. There are only nine named craters. Other features include those found elsewhere on other solid bodies in the system (and probably throughout the Universe): dorsa, sulci, catenæ (chains of craters caused by meteoroids breaking up before impact), maculæ (dark spots), pateræ (irregular craters, not the same as cavi), planitiæ and plana. There are also “regions”.

Tholins are present on Triton, where they are distinctive in containing heterocyclic nitrogen compounds. This makes them chemically similar to alkaloids, which are a family-resemblance defined class of nitrogenous compounds which tend to have rings containing nitrogen in their molecules, a markèd physiological effect on some organisms and originate in plants. However, there are animal alkaloids such as toad poisons and adrenalin, so it’s entirely feasible that there are basically drugs on Triton’s surface. Unlike Titan, there are no persistent solvents on Triton, so in a similar way to moondust being chemically different from matter in a wet or oxygen-rich environment, Tritonian tholins might be quite reactive on Earth, and might in fact be explosive. All this is my speculation, but I stand by it and feel quite confident that it would be so.

To conclude, then, probably less is known about Triton than any other body of comparable size in the system up to and including Pluto. It’s only been visited once, by Voyager 2, and was in fact the last world to be encountered by it before the “void”. Nonetheless, it’s an important world and has probably the best claim to planethood of any moon. The behaviour of objects in the outer Solar System at this point reminds me of snooker.

Next time, the other moons of Neptune, which are also interesting but even less well-known.

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.

Nine Planets Again?

Schlegel, Finkbeiner and Davis (1998)

Removed on request

In 2006 CE, the International Astronomical Union declared a new definition of “planet” which excluded Pluto because it didn’t satisfy the new criteria. These were:

1. It had to orbit the Sun (or presumably another star or it’s very silly).
2. It had to be almost round (so no doughnut-shaped planets?).
3. It had to have cleared the neighbourhood around its orbit.

They did this because a number of large new objects had recently been discovered which were round and two, I think, were more massive than Pluto, but they didn’t want to call them planets because it would’ve led to a very large number of bodies ending up being called that. They also introduced a new category of “dwarf planet”, which included Ceres, previously regarded as an asteroid, and also Pluto and others. It does make sense to do this, although I don’t understand why they didn’t just carry on with the term “minor planet”, referring mainly to asteroids, or perhaps “planetoid”, which they’d also used a lot.

The least clear of these three criteria is “clearing the neighbourhood”. This means that a body has no other bodies of comparable size other than its moons or other bodies under its gravitational influence such as Trojan asteroids. These are asteroids which orbit 60° ahead of or behind a planet in the same orbit which are pulled there by the gravity of the Sun and the planet concerned, examples being Achilles and Hector with Jupiter. Arguably this criterion either makes Cynthia a planet or Earth not a planet, and whereas I’m fine with the former I don’t think the latter is sensible.

The word “planet” has been applied differently during different times in the history of astronomy. When the large Galilean moons of Jupiter were discovered in the early seventeenth century, they were referred to as planets, and this also happened when Ceres was discovered in 1801. A similar process to the one leading to Pluto’s demotion then ensued, with lots more “planets” being discovered until it was decided to call them minor planets or asteroids.

It’s actually quite nice to think of Cynthia as a planet because it increases the number of known planets in our Solar System to nine again, and also means the Apollo astronauts landed on another planet rather than just a moon, and it also bolsters the idea that it should have its own name. It’s the largest body within the asteroid belt which isn’t considered a planet. Leaving that aside though, one issue with Pluto not being a planet is that most people have grown up with the idea that it is one, and it’s hard to let go of apparent certainties arrived at in childhood. Its demotion is akin to the youth of today liking different music or something. To quote Abe Simpson, “I used to be with ‘it’, but then they changed what ‘it’ was. Now what I’m with isn’t ‘it’ anymore and what’s ‘it’ seems weird and scary. It’ll happen to you!”. And it did. It happens to all of us.

I exploited this idea in my Caroline Era alternate history with the discovery of Persephone and subsequent visit by Voyager III. This body is in fact either Eris or Sedna, I can’t remember which. There is also an eleventh planet according to the Caroline Era astronomers, which is whichever one this isn’t, and this could’ve happened. It isn’t an alteration to the solar system, just to what we call things, and the name Persephone has been hanging around waiting to be attached to a new outer planet for a very long time now.

When Neptune was discovered, its mass and position explained some of the vagaries of the Uranian orbit but not all. Neptune also takes more than a gross years to orbit the Sun, so it was too slow-moving to plot its orbit accurately for quite some time after its discovery. Therefore, it was conjectured that a further planet must exist beyond the orbit of Neptune. Two planets were proposed, one by the well-known Percival Lowell who elaborated the Martian canals. He proposed a planet seven and a half times Earth’s mass with a mean distance of around 6 500 million kilometres from the Sun and a period of 299 years. It would have had a diameter of around 25 600 kilometres. Those figures, which turned out to be very wrong for Pluto, are worth remembering because they suggest something else, but I’ll be coming back to this. The other proposal was from Edward Charles Pickering. He suggested a planet with a mean distance of 8 200 million kilometres from the Sun and a period of 409 years. Obviously it couldn’t be both. Incidentally, this is where “Planet X” comes from. It was Lowell’s name for this planet while it was still undiscovered. Then, after a lot of searching using photographic plates to detect the movement of the body against the background of the stars, Clyde Tombaugh detected something moving in approximately the right position. After a competition, the eleven year old Venetia Burney decided it should be named Pluto, because it was far out, dark and gloomy and therefore appropriately named after the god of the underworld, which also happened to begin with Percival Lowell’s initials.

Both astronomers had predicted a highly elliptical orbit in comparison to the other planets, and in fact its orbit is indeed considerably more elliptical than any of them apart from Mercury, and was still quite a bit more eccentric even than that. For a long time, Pluto’s satellite Charon remained undiscovered due to being very close to Pluto in both distance and size, and consequently there was no easy way to calculate its mass, so it seemed that in order to yank Uranus around sufficiently from that distance it had to be practically a solid ball of iron, probably the densest element found in large enough quantities to make up an entire planet. If Charon had been found earlier, its orbital period would’ve indicated that Pluto was in fact not very dense at all and mainly made of ices, so when it was discovered in 1978, or more likely somewhat later when its month became known, it was realised that Pluto was not nearly massive enough to account for it. Its density is only 1.88 grammes per cm³ rather than more than four times greater as it had had to be assumed. So it looks like Pluto was actually just discovered by chance and has nothing to do with perturbing Uranus. Astronomers just happened to be looking really hard at the patch of sky it was by chance crossing at the time. It was in fact fainter than expected too, because they thought it would be larger, and the size of Pluto was also overestimated for a while for the same reason as its mass. In fact, to fulfil requirements it would actually have had to be more than twice as dense as the densest atomic materials in existence. Note that that doesn’t mean “known”. The densest elements are already known because the strength of the nuclear strong force compared to the other forces in atomic nuclei allows the heaviest stable elements to be determined, and they’ve already been discovered in the form of osmium and iridium.

Pickering believed that his planet and Lowell’s were not the same, and that both existed. To return to his “Planet P” as he called it, it’s of a type which is nowadays referred to as a “Super-Earth” or “Mini-Neptune”, and these are notable by their apparent absence from our Solar System. Of all the planets discovered in the Galaxy by the current rather flawed method, the most common of all are of this type: considerably larger than Earth and considerably smaller than Neptune and Uranus. It is in fact an unresolved problem in astronomy that the apparently most common type of planet also seems to be completely absent from our own system. Some have suggested that at some point a Super-Earth did indeed orbit with us but was slung out of the system entirely, or way too far out to be easily detected, æons ago, which is why we seem so atypical.

Before I go on to the next bit, I want to talk about Uranus and Neptune, both of which were “precovered”, i.e. noted before it was realised they were planets. William Herschel published his ‘Account Of A Comet’ in 1781, where he thought he’d found a comet but it turned out to be Uranus. This planet is actually just about visible to the naked eye and could easily be mistaken for a star. Neptune is too faint for this to happen, although I wonder if nocturnal animals can see it as well as Uranus, so the idea of it being discovered when it was may be preceded by perhaps 200 million years or more, although that would only be an early mammal happening to notice a light in the sky rather than a genuine discovery. It is, though, possible that Neptune was recorded as a star by various astronomers before it was actually found to be a planet.

And this brings us up to date, because as you probably know, a ninth (tenth‽) planet may have been discovered through old telescope photographs. The IRAS project, from a satellite launched in 1983, was an infrared sky survey operating for nine months. As seen highlighted in the image at the top of this post, it may have found a new solar planet. The object in question is in roughly the right place for Planet 9 but may not be a planet at all because it’s close to the galactic plane, where there’s a lot of dust and stars, making observations rather difficult. If it is a planet, it’s about 225 AU from the Sun (33 750 million kilometres or one light day and seven light hours from it) and has a mass at least five times Earth’s. If that difference is average it would take more than three millennia to orbit the Sun and the last time it was in the position it was in 1983 would’ve been in the late Bronze Age. It may well not be a planet at all.

The reason Planet 9 might exist is that the Pluto-like bodies orbiting between 150 and 300 AU out – those are average distances by the way and the orbits are far from circular – seem to be clustered on one side of the Sun but are too far out to have their movement disturbed significantly by the gas giants we know about, so the idea is that there is a planet even further out which influences their motion. Although I’m in the Dunning-Kruger zone with this, I have my doubts because it seems to me that the bodies we know about are all currently near their closest approach to the Sun because otherwise they’d be too dim and slow to be detectable, and it could be an artifact of a small sample size. I may well be wrong about this. If it exists, the planet in question would be about five times Earth’s mass, as stated above, but also 400 to 800 times further out than us as opposed to 225. However, Pluto was discovered because of looking in the right place accidentally, so although the hypothesised planet is too close, it doesn’t mean it isn’t there. Presumably it could mean there’s yet another one further out. Some people are uncomfortable calling it “Planet 9” because they see it as insulting to Clive Tombaugh. I feel a strong urge to call it Persephone. It isn’t the hypothetical Tyche, because that would be larger than Jupiter and has been ruled out by observation at any distance closer than 10 000 AU. Tyche would actually be fairly warm incidentally, because it would be large enough to heat itself – it would be only slightly cooler than Saturn.

A super-Earth at that distance, though, would be very cold. I’m not sure how cold exactly, but it would be between -270°C and -195°C. Planets of this type are either water worlds or “gas dwarfs”. At that distance it seems unlikely it would have oceans because they’d be frozen solid, but one depiction of a gas dwarf is that it would be like this:

By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=112487881

It could also have moons, which I find interesting because they could be warmed by tidal forces and if not, might have neon-rich atmospheres if they’re large enough.

The subject of Super-Earths and/or Mini-Neptunes is worth holding over for a post in itself, so I won’t go into more detail here, and I really think this is going to turn out to be nothing, but it’d be nice to discover another planet in our Solar System and perhaps resolve the problem of why we don’t seem to have one of this type. Alternatively, maybe a planet at that distance is far enough out to have been a rogue planet wandering between the stars or to have belonged to another solar system entirely which passed too close to the Sun and had one of its planets captured, which is exciting as well because it means we’d be able to study a planet from another star at relatively close range. It’s still over a thousand times closer than the nearest star though.

So to conclude, because good science always goes for the most boring option, I don’t think this is Persephone, but it’d be nice if it was.