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.
A Box Of Chocolates 