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.

A Solar System Oddity

It’s recently been asserted, with some evidence, that the Solar System may be an exception in certain ways. We have moved from the assumption of mediocrity, also known as the Copernican Principle, that there’s nothing remarkable about our solar system to the realisation that it may in fact be quite peculiar. Specifically, one of the weird things about it is that it consists of planets moving in roughly circular orbits with small rocky ones near the centre and gas giants further out. Also, the most common type of all the planets type is between the sizes of Neptune and Earth, and we don’t even seem to have one of those, although it’s possible that it’s orbiting too far out to have been detected so far, perhaps having been thrown out early on. Another common feature of solar systems, though probably an artifact of how exoplanets are detected, is the prevalence of “Hot Jupiters”: planets around the range of Jupiter’s size which are however very close to their suns and far hotter than any of the planets orbiting ours, with atmospheres of vaporised metal and clouds of what would be minerals on Earth. It’s been hypothesised that Mercury is a leftover of such a planet, although if it is, it’s surprising it didn’t disrupt the Solar System so severely that it destroyed or flung out most of the other planets.

What I have in mind today, though, is a bit different. It’s about the relative sizes and masses of the planets. It was noted in the mid-twentieth century CE that the planets had a trend of increasing size up to Jupiter and then decreasing to Pluto, when Pluto was considered a planet, the exception being Mars. This led to the Tidal Hypothesis, now discarded, that they formed when another star approached the Sun and pulled out an enormous filament which resembled a cigar or spindle, in that it was thin at one end, much much thicker in the middle and thin again at the other end, just like Anne Elk’s theory of the Brontosaurus which was hers.

This theory was replaced by the Nebular Hypothesis, originally devised by Immanuel Kant in the eighteenth century, which came back into vogue. Incidentally, Anne Elk’s theory of the Brontosaurus does actually count as a genuine theory, not just an hypothesis. It could be refuted by the discovery of a “Brontosaurus” (that name is deprecated) with a short neck or a “Manx” Brontosaurus without a tail, although it would have to be demonstrated that the tail, for example, was absent rather than missing due to such factors as predation or geology. Incidentally, Brontosaurus is now once again considered to be a valid genus, after going through a long period of doubt, so there is hope for Pluto yet.

Another notable aspect of the Solar System is the spacing of the planets, which also appear to obey a law. Taking the numbers 0, 3, 6, 12 and so forth and adding four to each accurately predicts the relative distances of most of the planets from the Sun. However, this could be coincidence because some of this is kludged. Neptune doesn’t fit into the sequence, Mercury corresponds to 0+4 and not really in the series either, Pluto does fit in but is no longer officially a planet and the approximate position of the asteroid belt, and more specifically Ceres, is correctly predicted but again the asteroids are not major planets. Hence there are up to four exceptions out of nine, considering Pluto as a planet but not Ceres, which makes the “law” a bit shaky.

However, what I want to concentrate on today is the oddity that both Uranus and Neptune and Venus and Earth are “twins”. I’ve mentioned the Uranus/Neptune issue already, though in a different setting. They are both quite similar in size and mass, and they also look quite similar, Neptune being bluer than Uranus and Uranus being hazier and blander-looking than Neptune. Neptune is 18% more massive than Uranus, which is less than it sounds because mass is somewhat related to volume, but is also considerably denser at 1.77 times water compared to Uranus’s 1.25, and in terms of diameter Neptune is five percent smaller. Turning to Earth and Venus, we are 22% more massive and five percent larger in diameter. Taking these four planets out of the picture, the two most similar planets in this respect seem to be Mercury and Mars, whose surface gravity is almost identical, but Saturn and Jupiter are not that similar, Saturn being quite serene and calm-looking (although I’m sure it isn’t) and Jupiter quite manic and boily. Uranus and Neptune are more similar to each other than Earth and Venus in terms of conditions, with similar colours, atmospheres and to some extent temperatures, although Neptune’s day is much shorter. Probably coincidentally, both Uranus and Venus spin in the opposite direction to all other planets, are the further planet in and are slightly less massive, although all of these are likely to be coincidental. Uranus is unusual in orbiting on its “side”, the axis being almost parallel to the plane of the orbit, and is technically retrograde but only just.

Two questions occur to me here. One is whether these two sets of twins are just coincidence or more significant, and the other is how common twin planets are in the Universe. I don’t fully know how to answer either of these questions although I kind of played with the idea in the post linked above. One thing which is notable is that both sets of twins are one and two orbits away from Jupiter, which would work well with the Tidal Hypothesis although that’s now been rejected. It might, however, reflect either a tendency for the solar nebula to bulge at a mid-distance and taper off closer to and further away from the Sun, or a tendency, which may be the same thing, for Jupiter to pull matter toward itself. However, the spacing of the outer Solar System is much wider than the inner.

Earth is obviously the object of more scrutiny than the others, and a couple of things should be noted about us. One is that we used to be more massive and bigger than we are now, since our planet collided with Theia, a Mars-sized body (and I can’t help wondering if it actually was Mars but I expect this has been considered and rejected) and chipped off an eighty-first of the mass in the form of our natural satellite, which is anomalous in size. Just adding the volumes together gives the original Earth a diameter of around 12 841 kilometres, makes it slightly less dense and slightly reduces the surface gravity. It’s very salient to the question of life elsewhere to consider how Earth would’ve turned out had this event not taken place, but right now I only want to talk about the likelihood of twins in a star system. Earth also has a year 11.86 times shorter than Jupiter’s, suggesting that the matter this planet is made of was pulled away from a zone either side of a dozenth of Jupiter’s year by continual tugging when the planet made its closest approach. Doing the same calculations with Uranus and Neptune, the former has just over seven times the period of Jupiter, closer in fact than Earth’s to an integer fraction, and the latter is around twice Uranus’s. Venus is not close to either Earth’s sidereal period (year) but is close to a third of that of Mars. It would be interesting if it turned out that Venus was able to win the gravitational battle with Jupiter to cause Mars to form, but not to the extent that Jupiter was able to disrupt any planet which would otherwise have formed from the asteroids plus a very large amount of extra mass which would’ve been necessary for a planet to form in what became the asteroid belt. However, although it’s feasible to do the maths for all these planets, the point comes at which mere coincidence would appear to stand out, particularly when one considers that all sorts of resonance ratios need to be considered.

It’s very easy to speculate and not very scientific to do so. Nevertheless, the patterns here seem to be that both pairs of twin planets are next to each other, one of each has close to a multiple of Jupiter’s orbital year and the other hasn’t and both are some way between the apparently most massive region of the solar nebula and the thin edge. There could be another reason why the biggest planet is in that location. Perhaps it’s simply that collisions between particles are more likely either to propel them towards the halfway point (which it isn’t any more, incidentally) or less likely to leave the solar system entirely, so there’s a build-up but not due to a thicker ring of material as such. Another, very important, factor, is that lighter elements, or those with lower boiling points, are likely to be driven off the centre of the disc and be retained the further out they are, which goes some way towards explaining the distribution of small and large planets but fails to account for Uranus and Neptune, as by this token they should be the largest if that’s the only or a major factor.

I’m very much in the dark here. I don’t think this has often been remarked upon. Venus and Earth have often been compared and contrasted, as have Uranus and Neptune, but the fact that this happens twice in this star system alone seems remarkable. All the planets involved are of intermediate mass, although Earth is the largest and most massive inner planet. There is a somewhat similar case with the star system TRAPPIST-1, with eight detected planets all between the masses of Mars and slightly more than Earth, and all in roughly circular orbits and closer to the star than Venus is to the Sun. This is somewhat extreme and unusual, but due to the small size of the star it might make sense to think of it as rather like a planet and its moons, similar to Jupiter and Saturn, more than a solar system like this one. Considering the moons of the outer planets, although the largest of Jupiter’s have somewhat similar size in terms of order of magnitude rather than being quadruplets, Saturn and Neptune each have one larger moon and many smaller ones and Uranus has two sets of twins, Titania and Oberon, and Ariel and Umbriel, although they are next to each other in that order outward. Saturn’s mid-size moons are all quite distinctive but often similar in size to others, so they can’t really be thought of as twins in the sense that Uranus and Neptune can, although Venus and Earth are substantially unlike each other apart from size and internal composition as well. Therefore, perhaps there are two trends, again reflected in our own system, of similar and dissimilar twins, and stretching the point somewhat, might this mean that there are similar and dissimilar twin planets elsewhere? That this is characteristic?

In particular, might there be twin mid-size planets in inner solar systems? The type of planet which isn’t in evidence in our own Solar System which is intermediate in mass between Neptune and Earth, somewhat dissimilar to each other owing to their closeness to the star seems highly plausible. Probably the cause of the differences between Venus and Earth by contrast with the rather similar Uranus and Neptune is that, being closer to the Sun, the temperature and radiation gradient is greater and their environments are therefore more different, leading to them being less similar.

Suppose, then, the following hypothetical situation. A planetary system has a super-Jupiter situated where our asteroid belt is relative to its own sun, making it the fifth planet, 2.8 times Earth’s distance from the Sun. I’m assuming it has to be larger in order for mini-Neptunes to form in the inner Solar System. These would then both be between the orbits of Venus and Mercury, and therefore both rather hot, though not as hot as Mercury, at least at the cloud tops. They would therefore have lost much of their light gases and shrunk in size, but would still be around 50% larger than Earth and Venus in diameter. However, being watery, both would probably still have runaway greenhouse effects. I’m not going to try to come up with a scenario where life could emerge, because this is a very common skew in how planets tend to be discussed. This is more to do with trying to illustrate the diversity of planets in the Universe.

Another possibility is a system where a Jupiter-sized planet formed at the distance of Saturn from the Sun, and incidentally like the previous example I’m trying to keep the model simple here by presuming the star has the same characteristics as ours. This could place two roughly Earth-sized planets where our asteroid belt and Mars are. The outer twin here is of a type absent from our system once again, possibly with liquid ammonia oceans and an atmosphere with some hydrogen. Water ice would never melt on this planet. There might also be formaldehyde mixed with ammonia in the oceans, making this planet hostile to life but very suitable for preserving biological specimens! The closer planet would occupy the orbit of Mars and be a “snowball Earth”, with conditions over most of the surface like those of Antarctica. In this case, life is possible around volcanic vents at the bottom of frozen over lakes of water, but the atmosphere would be largely nitrogen with dry ice on the surface. This assumes, of course, that the planet is unaffected by any filter, such as phosphorus availability, which would rule life out.

A final scenario to consider is the possibility of twin planets formed through the influence of a Hot Jupiter, further out from the star. A Hot Jupiter a tenth of Earth’s distance from the Sun could end up causing two medium-sized planets to form. It would itself have an eleven day year with frequent transits visible from those planets, which could be situated at about the distance of Mercury and about halfway to Venus. If they were about Earth-sized, the outer one would probably just be Venus-like, but the inner one might well have practically no atmosphere and therefore be heavily cratered, but otherwise Earth-like in size. This is again a planet unlike anything in our system.

All of this is highly speculative of course, but the main point is to illustrate that there might be many “twin worlds” out there about which we know practically nothing, all very different from anything in our own solar system. But as a concession to the fixation on Earth-like planets, it’s also possible to envisage a pair of worlds whose mean distance from their Sun is the same as Earth’s. The inner twin could be like the classic, golden age sci-fi version of Venus, a steamy, hot jungle planet permanently swathed in water vapour clouds with heavy rainfall, and the outer could be a chilly version of Earth, with Arctic and Antarctica conditions but maybe conditions in the tropics comparable to Scandinavia. This could well be a star system with two habitable worlds, and perhaps two worlds with Earth-type life on them.

There is another way of getting twin worlds, which might be called “conjoined twin worlds”. Earth was split into two bodies by the Mars-sized Theia. A larger planet-sized miscreant might have split our planet into two roughly equal-sized planets orbiting each other. The difficult thing to manage here would be the distance between the worlds, as if they were at the same distance as our own double planet system, their rotation period would last several weeks and temperatures would fluctuate between conditions which would boil the oceans and conditions which would freeze them solid, so this would be a nasty pair. However, if they were quite close, but not close enough to tear each other apart, they would form two smaller, more arid and mountainous worlds with less water but deeper oceans. These would then be desert worlds, perhaps with deep lakes rather than oceans, and mountains reaching high above the cloud tops, which would in any case be lower than on Earth, perhaps with whole plateaux above them where it neither rains or snows. However, the mean temperature at a given latitude could still be comparable to ours. But there could equally well be double Veneres or Martes, and in the latter case it would likely be a pair of cold Mercuries.

To conclude then, I think if we get to adequately explore the Galaxy, evidence from this star system strongly suggests that there would be plenty of twin planet situations, and as far as I know this has never been explored theoretically by astronomers. Nor, so far as I know, has the fact of there being a pair of twins here been investigated. I’ve used a fairly naïve model to imagine the planets here, but even if I’m wrong, and I probably am, I still think that there are likely to be many twins in the Universe, and I look forward to some being discovered.