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.

Restoring Pluto And Elevating Cynthia

I was going to blog about the larger asteroids at this point, but in recent days it’s been borne in upon me that there’s a current issue in astronomy, perhaps over-emphasised but definitely there, over whether Pluto was unfairly demoted. The reason I mention this now is Steve’s comment about what the difference between Phobos and Deimos and asteroids might be. It’s a very good question and I’ll address this first.

Phobos and Deimos, the moons of Mars, are a little puzzling. There are two hypotheses about where they come from. One is that they’re main-belt asteroids which were captured by Mars. At first glance this sounds very sensible and logical. After all, Mars is next to the asteroid belt, it could be expected to gather up a few stones from it from time to time and the pair seem to be only the latest representatives of a whole series which have scarred Mars with chains of craters as they broke up and impacted. However, there are problems with it. Firstly, the common type of asteroid found near the edge of the belt closest to Mars is different from the type of asteroid Phobos and Deimos would be if they are asteroids. That type is found near Jupiter. This is due to the inner belt being warmer than the outer belt, so the composition differs because temperature makes a difference to them. Secondly, both moons have almost perfectly circular orbits over the Martian equator, and if they were captured, they would usually have come in at a high angle to the equator and have markèdly elliptical orbits. This can be seen with Nereid, Neptune’s third largest moon, and Saturn’s moon Phoebe orbits backwards compared to most other bodies in the system. Therefore, if Mars’s moons are asteroidal in origin, something needs to be evoked to explain that. A simpler explanation would be that they emerged from the cloud which was forming Mars. This would be spinning in the same plane as any moons which formed from it, and if they were formed in situ they would be more likely to have almost circular orbits. However, as Steve astutely pointed out, the actual nature of the bodies themselves is very close to being asteroidal, and in fact is asteroidal, so maybe it doesn’t matter in most ways. In the sense of the physical nature of the two moons, they basically are asteroids. The way in which they aren’t is to do with their history and orbits, which may not be a sensible thing to focus on. The only thing which goes against this is that both are directly affected by orbiting Mars. Phobos has streaks because of the tidal forces of its planet, and Deimos accumulates fragments and dust from itself as it moves through its rather short orbit. If they were orbiting in the asteroid belt itself, neither of these things would be happening. All that said, I can totally see the argument that they are in fact just asteroids in an unusual place which are also moons rather than minor planets. So I agree with you Steve.

This connects to a wider issue which affects Pluto, and it also affects a number of other worlds in the system which if addressed could solve the problem of knowing what to call the big round things in our Solar System. It could also address the peculiarity of our own “moon”. The 2006 CE definition of a planet by the International Astronomical Union is:

The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.

This definition was motivated by the discovery of a number of relatively large trans-Neptunian objects. Eris, discovered at the start of the previous year, has now been established to have a diameter of 2326 kilometres and a mass of 1.6466 x 10²² kilogrammes. Sedna, discovered in 2003, has a diameter of around a thousand kilometres and an unknown mass because unlike Eris it seems to have no moons. Sedna is less of a threat to the status quo but Eris was initially thought to be larger than it has now turned out to be. For comparison, Pluto is 2376.6 kilometres and it has a mass of 1.303 x 10²² kilogrammes, so it’s actually slightly larger than Eris but also less massive, so the question arose of whether it would be acceptable to admit a potential host of newly discovered planets, thereby reducing the “specialness” of planets, or to invent a new category. This last idea, of “dwarf planets”, seems very odd to me because the category of “minor planet” had existed for a very long time up until that point and instead of inventing an entirely new class of object, it would’ve made more sense, if they were going to do this. Whether or not I agree with the decision, there seems to be no merit in creating a whole new category of “planet” when “minor planet” already existed. I honestly don’t know why they did this.

Many people have disagreed with the decision to demote Pluto. It did elevate Ceres, previously considered a mere asteroid, at the same time. Before that point, for most of its history since discovery Ceres was considered an asteroid, but it’s the only body in the asteroid belt which has managed to make itself round due to its own gravity (there might be other bodies which just happen to be round-ish through chance because asteroids are irregular and could hypothetically be many shapes, including spheroidal), so it probably does deserve special recognition.

In spite of this definition, which is quite unpopular, a paper has recently been published on the subject arguing that Pluto, among other worlds, does in fact merit planethood. The paper can be found here. It’s sixty-eight pages long and I haven’t read the whole thing but the general gist of it seems to be that there used to be a scientifically arrived-at understanding of what a planet was, but over a period in the early twentieth century when astronomers focussed more on what was happening outside the Solar System, the popular uneducated public understanding of what a planet was took over. I have to say this doesn’t reflect my perception of what happened based on my knowledge of astronomy. I’m aware of the controversy about the canals, the discovery of Pluto, the idea that Mercury always faced the Sun and so on, all ideas which resulted from astronomical research at around that time. I’m aware of the research that was being done at the time about stellar evolution and the realisation that there were other galaxies, but it really doesn’t seem like they were concentrating that much on that more than this Solar System, but anyway, that’s what this paper claims.

Further, it claims that because they adopted a kind of folk understanding of what a planet was, it had led to them adopting earlier, non-scientific ideas about it. So for example, the public was really into astrology and had only recently got used to the idea that the Sun was at the centre of the Solar System rather than Earth. The authors of the paper give examples of how scientific classifications differ from public ones. For instance, most people think of fruit and vegetables as two different things but when it comes to botany, vegetables include fruits, which are the reproductive organs of plants, so from a culinary viewpoint fruit and veg are separate but scientifically they aren’t. To this I would add a couple of things which are I hope relevant to astronomy. One is that I think of a lot of things as fruit, such as tomatoes, aubergines, courgettes, peppers and tomatoes, which other people seem to think of as vegetables because it makes sense to me to think of them nutritionally and in terms of flavour in that way. The other is that the culinary arts are also sciences, and it seems a bit hierarchical to see them as inferior to botany for some reason. After all, we all need to eat. Applying that to astronomy and planets, that would mean that although some things are planets and some things aren’t according to astronomers of a particular vintage, that doesn’t mean there isn’t another branch of science which would view them differently. For instance, everything is subject to the laws of physics, and geology would seem to apply pretty much equally to planets, moons and asteroids in their own way. They’re just bodies in space like everything else. Therefore, I’m not convinced about this. Also, the general public were specifically irritated at the idea of Pluto not being a planet any more, so I don’t see how exactly they were using the public view of what planets were if they managed to annoy so many non-astronomers with their assertion that Pluto wasn’t one.

What seems to have happened is that the problem crept up on astronomers and they kind of panicked and made a fairly slapdash and hasty decision. As various large bodies were discovered on the edge of the Solar System, they became uncomfortable with the idea that they were probably going to end up with a very long list of planets, which seemed unwieldy and not very “neat”, and they also perceived it as an imposition on education that people were going to have to learn about so many worlds. They seemed to feel like this would be regarded as off-putting. The paper compares the situation with how mammals are defined. The official definition of a mammal is now rather abstruse, because it actually hinges on how many bones are in the jaws and the ears, but this is partly because of the need to identify fossil mammals. The widely-used definition is “animals who suckle from their mothers as infants, maintain a different body temperature from their environment, are often covered in fur or hair and mostly give birth to live young”, and the first criterion is the most important. There are exceptions to most of these. For instance, some hibernating mammals don’t keep their body temperatures above their surroundings and humans, whales and elephants are largely hairless, but this is a fairly good definition. However, claim the authors, astronomers have taken a weird approach to planets, having concentrated on whether they dominate their local region, which is in any case vague because what’s local? They’ve also looked at how they move. If mammals had been defined in this way, as warm-blooded vertebrates who walk in herds on land for example, a lot of mammals would’ve been excluded. Bats and whales would then not be mammals and any mammal who has a largely solitary life, such as leopards or sloths, would not then count as mammals either.

Looking at the history of the idea of planets, for a long time any round object in the sky which didn’t appear to stay in the same place was a planet. This used to include Cynthia and the Sun, when people thought Earth was at the centre of the Universe, and it didn’t include Earth. Later on, the four largest moons of Jupiter were discovered and also referred to as planets, and even the thick parts of the rings on either side of Saturn due to the poor quality of telescopes at the time. Later still, Ceres was called a planet because it seemed to fit into Bode’s Law, and turned up where it was expected. By that time, however, the known satellites had been relegated to moons, and soon after Ceres was also demoted because it was realised that there were thousands of other bodies between Mars and Jupiter, some even quite large.

The 2006 definition also has a rather silly consequence which a few people have noticed: it means Earth isn’t a planet! As I’ve mentioned before, from the Sun’s perspective Cynthia doesn’t orbit Earth, but the two weave in and out of each other’s orbits. I’m not completely clear what the astrological influence is supposed to be, but I think it’s the emphasis on orbits, i.e. the kind of definition which would’ve excluded bats, whales and leopards from being mammals. Whatever the definition of a mammal is, it seems to make more sense to use their anatomy and physiology than other more dubious criteria. Both of the definitions I mentioned above do this. The first is rather abstract and strange to most people, although there are good reasons for it – mammal jaws and teeth survive better than the rest of their bodies so it’s like identifying a body by dental records – but both of them focus on what their bodies are like, which seems entirely sensible compared to that fictional other definition.

What, then, is proposed as a more sensible definition of a planet? Well, it’s closer in spirit to that way of defining a mammal. A planet is a geologically active body. I have to admit I’m not sure about this because of various things, such as “eggshell planets”, and I’d also want planets to be round and I can’t tell if they also stipulated that. What it means (I’ll get back to eggshell planets in a moment) is that Pluto’s Sputnik Planitia which is created by frozen nitrogen and is active even though the Sun isn’t strong enough at that distance to have that effect. In talking about asteroids, I’ve mentioned the fact that the larger ones tend to be layered like Earth is, but the smaller ones are either rubble piles or mixtures of different minerals and other substances which aren’t separated out in the same way. A geological process has done this sorting in the larger ones, and consequently Ceres, for example, could count as a planet: it has been geologically active.

This applies also to some moons. Io, the innermost large moon of Jupiter, is intensely active with continual volcanic eruptions, to the extent that it’s thought to “turn itself inside out” every few years – some much of its interior is spewed onto the surface that the former surface becomes the interior and proceeds to get thrown out itself a few years later. This is because of the tidal forces effectively “wringing” the moon all the time, with the other large moons in the Jovian system along with Jupiter itself wreaking havoc on the place. By this standard, Io is definitely a planet, albeit a planet which is also a moon.

I’ll now permit myself a digression into eggshell planets. An eggshell planet is a surprising kind of “planet” which kind of “does nothing”. It isn’t necessarily possible to tell from a distance which planets are like this. Earth’s crust is divided into plates, and other planets have a thick, solid layer all the way round, but there is another possibility or which at least three examples may have been found already. This is where the crust is thin and fragile, and so cannot have plates or thick layers, and also can’t even support mountains or hills, so the surface is solid and also smooth, and nothing happens there – no volcanic eruptions, continental drift or erosion, because there’s nothing to erode. The question arises of whether this even counts as a planet under this new definition, since it isn’t geologically active. However, there are no such planets in our Solar System as far as anyone knows, and they’re probably quite rare.

There are three categories of planets suggested in this new definition: terrestrial planets; giant planets; satellite and dwarf planets. The last category is the largest. It includes the large moons of Jupiter, Ceres, Titan, Pluto, Charon, Eris and Sedna, and in fact there are more than a gross of these. Far from the expected response, apparently people tend to be quite excited at the idea that there are so many planets around the Sun. The giant planets include Jupiter, Saturn, Uranus and Neptune, so no surprises there, although this clear-cut division may be an artifact of how our own Solar System is, with its complete absence of the very commonest type of planet, the mini-Neptune, intermediate between Earth and Neptune in size.

There are five planets in the terrestrial category rather than four, because once the criterion for dominating its orbit has been removed, Cynthia becomes eligible, which makes me very happy! Cynthia is not even in the same group as the satellite and dwarf planets, but a planet just like Mars and Mercury. This also means that the Apollo astronauts landed on another planet, not just our moon. As well as that, Earth now has no moon!

It seems that the process leading to the decision to redefine planets was not very scientifically grounded and was in fact rather acrimonious. The orbital dynamics people took umbrage at the geophysical definition and there were only a few days available for debate, forcing people to take sides quickly without due consideration. Planetary scientists were underrepresented because they’re apparently not officially astronomers, which is a bit astonishing. Another motivation was to keep the number of official planets low because the IAU didn’t expect the alternative to go down well with the public because previously, i.e. in Victorian times, they’d felt more comfortable with a small number of planets. They were used to seven at that point, including the Sun and Cynthia. This is probably no longer the case, so in 2006 they made a decision based on misjudging the mood of the general public.

To finish, I’m going to make a commitment. Henceforth I will be referring to every spheroidal body in the Solar System as a planet, although I will also acknowledge what kind of planet it is, such as a moon or dwarf planet. And Pluto is a planet!

I’d be delighted to hear your views on this.

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.