Are We Out In Dullsville Now?

If you go back to where I started this series properly, you’ll find that I produced a post, whose name and location I’ve currently forgotten, introducing the Solar System from the outside in. I’ve now returned to the outermost part of the system except for the Oort Cloud, and I ask myself, are these outer reaches really dull? Well, they are in a literal sense of course, in that the Sun is pretty dim at this distance, but the wide separation, small size and low temperature of worlds, if that’s the right word for them, combined with the facts that nothing has ever visited them and that they’re hard to detect, means that they might also be exceedingly boring. I can imagine people travelling to them who want to get seriously away from it all, and from other people. In fact, there’s a scene in an Iain M Banks novel about someone who has done precisely that. I think it’s ‘Excession’.

There’s a lot going on in the regions near the Sun, and I use “near” quite loosely as I intend for it to apply to Jupiter and Saturn, the latter being well over a milliard kilometres from it. Incidentally, why is it we get stuck at kilometres? I’ve just fished out an obscure English word to describe a distance which could easily be referred to as a terametre, and yet we never say that. The further out one goes, the less is happening, with the occasional exception such as Triton’s liquid nitrogen geysers and the mysterious brightness of the surface of Eris. Average distances between worlds increase, temperatures plummet and the Sun looks ever dimmer. That said, it’s still possible, for example, to imagine a world so cold that it has oceans of helium II which crawl over its surface and climb mountains, or outcrops of superconducting alloys which generate incredibly powerful magnetic fields. I don’t know if either of those things are possible, because the 3K background temperature of the Universe might rule them out and helium only becomes superfluid at 2.17K, but there have always been surprises. Few people would’ve guessed that Neptune has winds which blow faster than the sea level speed of sound, for instance. Perhaps high winds on a very cold planet would cool it below the temperature of deep space.

Considering the history of the Universe, a frantic and hyper beginning slows down continually, through the current stelliferous era and other less and less eventful stretches of time until basically nothing is happening. Space is rather like this too. Not a lot goes on in the Oort Cloud.

Even so, there is stuff out there. For instance, there’s a planetoid nicknamed FarFarOut, which is 132 AU from the Sun. Also known as 2018 AG₃₇, FarFarOut is about four hundred kilometres across, which means it could be round. It actually swings round to being only 27 AU, closer than Hamlet. It takes 718 years to orbit and at its maximum distance of 132.7 AU the Sun is almost 18 000 times dimmer than from here. There’s also 2019 EU₅, which averages 1 380 AU from it and has a maximum distance of 2 714 AU. These figures are highly uncertain, but if the aphelion is correct (it could be considerably greater or less), sunlight at such a distance is finally weaker than our moonlight and the planetoid takes fifty-one thousand years to orbit the Sun at a mean velocity of about eight hundred metres per second. With such planetoids, it becomes difficult to judge their actual trajectories because they move so slowly and haven’t been observed for long.

There are now five human-built spacecraft out there: Pioneers 10 and 11, Voyagers 1 and 2 and New Horizons, the last being the newcomer, only launched in 2006. Voyager 1 was manœuvred out of the ecliptic so it could get a good view of Titan, and is therefore heading out into the scattered disc rather than the Kuiper belt. It’s 153 AU from the Sun at the moment. Voyager 2 is 130 AU out. Both were launched in 1977. The Pioneer probes have been going for rather longer but are actually closer, at 129 and 108, but they’re all now over twice as far away as Pluto ever gets. New Horizons is a mere 50 AU from the Sun right now. Now a viable claim is made that the Voyager and Pioneer probes are now in interstellar space because the pressure of the solar wind is weaker than the ambient “flow” (I suppose) of charged particles between the stars, but there are still planetoids orbiting out there, even ones which never dip into the volume inside the heliosheath. Isaac Asimov’s novel ‘The Currents Of Space’, though its science is out of date, uses the idea of similar flows as an important plot point, so this is one possible way in which the outer part of the Solar System might not be boring. Processes taking place within the heliosheath which influence planets, asteroids, moons and so forth would not operate beyond it. For instance, any magnetospheres which exist out there would not be thrown into asymmetry by the solar wind, and larger and denser atmospheres could exist out there, although the only elements able to maintain a gaseous state at such temperatures would be hydrogen and helium, and in fact ultimately helium. It also means the useful isotopes found in lunar regolith would be absent from many trans Neptunian objects and this reduces the utility of mining for them.

There are a dozen known planets, dwarf planets by the IAU definition of course, which reach 150 AU or more from the Sun. This is one motivation for not calling them planets. If they were, they’d now outnumber the major planets. The same is, though, also true of asteroids and centaurs, and asteroids were simply called “minor planets”. The whole thing seems a bit silly and solves a “problem” which had in any case already been sorted when such concepts as major and minor planets, or planetoids, were invented to address the issue after the discovery of Ceres, in the early nineteenth century CE. Right: I’m going to resolve not to go on about this for the rest of this post as I’m sure it’s getting old. These objects include Haumea, Quaoar, Eris, Sedna, Makemake, Albion, Gonggong, Pluto itself, Varuna, Arrokoth, Arawn, Chaos, Ixion and Typhon. Others are also named, but most don’t come up much in discussions or news, and most of them have provisional designations. To be honest, some of them just stick in my mind because of their names, particularly Quaoar but also Makemake and Gonggong. FarFarOut has a predecessor which isn’t so far out called FarOut. There are two zones: the Kuiper belt, which consists of objects orbiting near the plane of the inner system, and the Scattered Disc, comprising objects whose orbits are more tilted. The second category developed because of the gravitational influence of the outer planets, although it occurs to me that this might also be the region where the Sun’s influence and the traces of the solar nebula become less relevant to them. There is also a third region, the Oort Cloud, which is in really deep space beyond either of the others, whence some comets originate, and extends for over a light year in every direction. TNOs are also distinguished by colour (Eris springs to mind but that’s a special case as far as I know). They’re either steely blue or bright red. A classification kind of cutting across this are the poorly-named “hot” and “cold” categories. Cold TNOs orbit close to the ecliptic and are usually red. Hot TNOs have tilted orbits and range between the two colours, which means that the red ones are the “cold” ones.

By Pablo Carlos Budassi – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=94143935

One of the weirdest known trans Neptunian objects is Haumea, illustrated above. This has three remarkable features. It has a ring, two moons and is ellipsoidal but far from spherical. It counts as a dwarf planet. Its unusual shape is called a Jacobi ellipsoid, and is rather surprising. It intuitively makes sense that a rapidly-spinning body would be thrown outwards at its equator and therefore assume a kind of tangerine shape, or perhaps even a discus shape, as seen clearly with Jupiter and Saturn but also with most major planets including Earth to some extent. Venus and Mars are somewhat different, the former being almost spherical and the latter having a more egg-shaped form due to the Tharsis bulge. This more intuitive shape, an oblate spheroid, is quite common and the torus is another quite remarkable stable shape which, however, is hard to envisage actually forming in the first place. There is a notorious (to Sarada and me) pebble classification system called Zingg (two G’s), which divides them into spheres, discs, rods and blades according to their X, Y and Z axes. This used to be a source of joy to us due to its apparent obscurity, but has its uses, and Haumea counts as a blade. Each axis is markèdly different to the other two. Lagrange, who discovered the points of gravitational equilibrium around pairs of masses responsible, for instance, for the trojan asteroids in the orbits of several major planets and the trojan moons in the Saturnian system, held that the only stable shape for a rapidly rotating body of a certain size was the oblate spheroid, but counter-intuitively, this turns out to be wrong. This is the gateway to a whole branch of geometry involving ellipsoids.

Haumea’s axial dimensions are 2 322 × 1 704 × 1 138 kilometres. It spins once every three hours and fifty-five minutes, which is particularly high considering its size. Comparing it to Pluto, for example, that planet takes six and a half days to rotate and has a diameter of 2 377 kilometres. Not only is Haumea considerably smaller and less massive but it also spins three dozen times faster, causing a much stronger centrifugal effect. I have to admit that not only is it entirely unclear to me why Haumea is this shape beyond the simply fact that it’s spinning really fast and has thereby had projections drawn out from it, but also I can’t understand the maths behind it. If this can happen once, maybe there are larger planets out there somewhere with the same shape, maybe even Earth-sized ones. It seems unlikely, at least because a larger object would tend to be more spherical, although there could be other reasons why it might happen such as a nearby massive body pulling it out of shape. Haumea was probably hit some time in the past by something which sent it spinning wildly. It also isn’t clear that it’s reached hydrostatic equilibrium although it’s very large for a solid object if it hasn’t.

Haumea is the Hawaiian goddess of fertility and childbirth. The planet’s moons are named after her daughters, Hi‘iaka and Namaka. It’s thought to be rocky with a surface layer of water ice and seems to have a red crater near one of the geometric poles (i.e. on the equator). I’m guessing the reddish colour is due to tholins. Haumea seems denser than most other Kuiper belt objects, including Pluto, and may be as dense as Mars or Cynthia. It has crystalline water ice on its surface even though its temperature ought to cause the ice to become glassy. There may also be clay on the surface, and cyanides of various kinds. Hence the very surface would often be highly poisonous to ærobic life forms, including humans. There is no methane, suggesting that it was boiled away in the heat of impact.

The ring spins once every twelve hours, in other words a third as fast as the planet. The moons are small and probably result from the collision. Another thing which probably results from the collision is the Haumea family. In other parts of the Solar System, there are various families of objects, for instance the Vesta family, which consists of Vesta plus the asteroids which have been chipped off it, including some meteorites which have arrived on Earth. The Haumea family is the only identified group of objects beyond Neptune, and originates from the collision. They’re all water-ice at the surface and are fairly bright. Some may be up to seven hundred kilometres in diameter and count as dwarf planets in their own right. They average between forty-one and forty-four AU from the Sun. One of them seems to be in the family but is red.

Haumea itself is 43 AU from the Sun on average and has an orbital eccentricity of a little under 0.2. It takes 283 years to traverse this orbit, so it isn’t enormously further away than Pluto and in fact it gets closer to the Sun than Pluto does.

Another name which sticks in the mind belongs to the dwarf planet Sedna. This is one of the reddest known objects in the system and is also tied with Ceres in being the largest moonless dwarf planet. Sedna is one of those planets which makes me wonder whether it’s one of many undiscovered ones, because it was discovered due to happening to be almost as close as it gets to the Sun at 76 AU. Even that distance is almost twice Pluto’s. It takes 11 400 years to orbit the Sun and gets out to five and a half light days from it. The last time it was there, there were mammoths on this planet and the pyramids had yet to be built. It’s around a thousand kilometres in diameter, like Ceres. It’s named after the Inuit goddess of the sea and its denizens. The extremely elongated orbit, which has an eccentricity of almost 0.85, could be explained by the presence of an extremely distant and large planet. It’s part of a class (as opposed to a “family”, as in the Haumea family) of objects whose perihelia are greater than 50 AU and mean distances over 150 AU from the Sun. These orbits have an eccentricity of around 0.8, so although that’s the definition, in actual fact they’re considerably more elliptical. It’s been established that there are no large planets in the system beyond Pluto to a considerable distance, although there is the question of a missing ice dwarf. That would, however, not be detectable by current methods and wouldn’t explain the sednoid bunching of orbits. It’s also been suggested that the sednoids move thus because they were influenced by nearby stars back when the Sun was young and part of a cluster of baby stars. There are occasional stars which seem to be almost twins of the Sun due to similar proportions of heavier elements (often referred to in astrophysics as “metals”), suggesting that they were once our companions. Alternatively, they may have been captured from those stars early on in the history of the system. The other two objects falling into this category are Leleakuhonua and 2012 VP₁₁₃.

As well as the usual tholins, Sedna is covered in frozen nitrogen and methane, which is present generally but absent from Haumea, probably due to the collision. Its orbit looks like this to scale:

By Tomruen – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=60453344

There may be amorphous carbon on the surface. Unfortunately the term “amorphous carbon” is ambiguous as it can mean charcoal- or soot-like carbon, which in fact consists of graphite sheets haphazardly arranged, or it can literally mean amorphous, i.e. glass-like, carbon, which might have special properties such as being a high-temperature superconductor and being harder than diamond. I suspect they mean the former – just a load of boring old black gunk like you might dig out of a coal mine.

Sedna is special because it isn’t. It’s probably an example of a very numerous class of objects orbiting way out beyond the influence of Neptune in the Oort Cloud. We happen to know it’s there but there are likely to be many, many more examples way outnumbering the objects known in the inner system whose orbits haven’t so far allowed us to detect them. That said, the presence of tholins is related to the influence of solar radiation so it might not be typical of them.

Another planetoid is Arrokoth, unique in being the only trans-Neptunian object other than Pluto-Charon and their moons to have been visited by a space probe, New Horizons. It was nicknamed Ultima Thule, but this was later deprecated due to the association with Nazi occultism. It was actually named in a Pamunkey ceremony. The common “dumb bell” appearance shared by two of Pluto’s moons, some comets and other objects is also seen here. It’s thirty-six kilometres long altogether but consists of two smaller fused planetesimals, fifteen and twenty-two kilometres in length. Planetesimals are the bricks which make up planets and moons, and have never been seen in their raw form before. If a twenty-kilometre object is typical, Earth would be made up initially of over a hundred million of them, having long since melted together and lost their identities. There are interesting substances on its surface, including methanol, hydrogen cyanide and probably formaldehyde-based compounds and complex macromolecules somewhat similar to those found in living things. The basin in the foreground, which is probably a crater, is a bit less than seven kilometres across and called Sky. The axis of rotation passes through the centre of the dumb bell.

Arrokoth is a “cubewano”. These are named after their first discovered member, 1992 QB₁. Also known as “classical Kuiper Belt objects”, cubewanos are often in almost circular orbits less than 30°from the plane of the Solar System, but are also often not. They have years between 248 and 330 times ours, the lower limit being defined by the plutinos with their sidereal periods close to Pluto’s. I’ve mentioned them above. They’re distinctive in not being particularly distant (relatively) and also not having orbits connected to Neptune’s.

Quaoar is a particularly large cubewano. Its name is from an indigeous people called the Tongva in Southwestern North America, although for a time it was called “Object X” as a reference to Planet X and because its nature was unknown. You can see the planetary definition crisis developing here, as it was discovered in 2002. It was first imaged in 1954, but like many other bodies went unnoticed for many years. It takes 289 years to orbit the Sun and is 43 AU from it. It seems quite dark, suggesting that it’s lost ice from its surface, which has a temperature of -231°C. It has a moon to keep it company, like many other trans-Neptunian objects. The diameter is around 1 100 kilometres.

Previously, the largest known TNO was Varuna, discovered in 2000. This may also be a “blade”-shaped planet like Haumea, and is just barely beyond Pluto’s average distance from the Sun at 42.7 AU, taking 279 years to orbit. It seems to be less dense than water and its average diameter was recently estimated at 654 kilometres. It takes six and a half hours to rotate on its axis.

I feel that this series is now drawing to a close. However, there are many objects I haven’t considered, such as the Neptune trojans, the possibility of Nemesis and the question of what large objects may be swimming out there in the depths of the Oort Cloud. There is also one planet I haven’t given its own post. It’s a small blue-green planet, third from the Sun, and will form the subject of my next post.

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.