Pluto used to serve as a metaphor for the back of beyond, somewhere so distant and obscure that it was hard to imagine anywhere more remote. In ‘Not The Royal Wedding’, Leonid Brezhnev’s share of the wedding cake was described as the size of “a microbe’s frisbee seen through the wrong end of a telescope well beyond Pluto”. Sadly, nowadays ‘Not The Nine O’Clock News’ itself approaches that level of obscurity. Also nowadays Pluto has ceased to be quite as effective a symbol of distance and obscurity as it once was, for two opposite reasons. One is that it isn’t a planet any more. The other is that it’s been visited and is therefore fairly well-known. However, there are still places in the Solar System which are far more distant and obscure even than Pluto, which is one of the reasons Pluto is no longer considered a planet.
Although it may be annoying from a nostalgic perspective, it does in fact make sense for Pluto not to be a planet. According to the newer definition, a planet is a body whose gravity is strong enough to make it approximately round, orbits a star and has cleared its orbit of debris. Pluto has not done this last bit, and the problem is not so much Pluto as that there are now known to be several other objects around the same size and mass which if they were also regarded as planets would strongly suggest that there are in fact hundreds of planets orbiting the Sun, which would mean being a planet was no longer special, or even meaningful. One of the sad things about this is that there are no new planets.
As a child, I liked the idea of the Solar System being symmetrical. There are four small, solid planets near the Sun, then an asteroid belt, then four gas giants, then Pluto. I wanted there to be another asteroid belt plus four more small, solid planets for the sake of neatness. At the time I believed in the “cigar” theory of the formation of the Solar System, which was the idea that another star had approached the Sun early in its history, pulled out a sausage-shaped projection, and that that projection had then condensed into the planets we see today, with the biggest one in the middle and smaller ones at either end. This theory was popular in the 1930s but I think by the time I was born it had been rejected and people had gone back to the Nebular Hypothesis, as conceived by Immanuel Kant in the eighteenth century. It would have meant that solar systems were rare in the and failed to explain why Mars bucked the trend of increasing size, so this was addressed by supposing that Mars, Earth and the moon were all formed from the same original body, something which I still suspect is true even now because Mars is much less dense than the other inner planets, being about the same as Cynthia/the Moon.
Pluto was always problematic. The existence of a planet beyond the orbit of Neptune was originally suspected because something seemed to be pulling on both Uranus and Neptune which altered their orbits and meant they were not where they were expected to be. The trouble was, although astronomers did find something, for it to work as the cause of the orbital perturbations it would’ve had to have been made of something like solid iron. At the time, Pluto’s mass was unknown because it would be another fifty years or so before its large moon Charon was discovered. When Charon was found, the fact that it took about six days to orbit Pluto, or more accurately, six days for Pluto and Charon to move in two ellipses relative to each other, at a distance of about 20 000 kilometres, meant that both had to be mainly made of ices. It just didn’t have enough “oompf” to pull Neptune and Uranus around.
It did in fact turn out that quite a few small worlds past Pluto still orbit the Sun. These include Quaoar, Eris and Sedna, plus a load of others which just have serial numbers. This diagram shows them in relation to the sizes of Earth and Cynthia/the Moon. Eris is the one closest to being a planet, ignoring Pluto, and also the reason it was decided Pluto wasn’t a planet any more. It’s about twice as far away as Pluto and around 2300 kilometres in diameter. It has its own moon, but this doesn’t make it more like a planet so much as less like one, because what with the array of relatively tiny rocks orbiting several of these objects the implication is more that they can’t even hold themselves together and are just accompanied by fragments of themselves, as if they’re piles of rubble. Pluto is slightly larger than Eris but Eris is more massive. Anyway, this kind of redefined the concept of “the back of beyond”.
This is a view of our Solar System looking from the North towards the Magellanic Clouds, which are satellites of the Milky Way. Earth’s orbit is too small to represent properly in this image, and the extra names are of southern constellations such as Dorado and Horologium, the Dolphinfish (not the Dolphin, which is a completely different constellation (and animal) and the Clock respectively, fairly obscure to those of us who look north into the sky at night and see Orion and the two bears. Most of the larger visible orbits here are of Kuiper Belt objects, which include Pluto, Eris and in fact most of the other worlds on that chart. Eris’s year lasts nearly six centuries, so it is indeed pretty distant. The Kuiper Belt consists of thousands of objects, most of which are currently unknown, and bearing that in mind, it does provide the symmetry I craved as a child of there being a second, outer, asteroid belt, and also gives a pretty good reason for Pluto not being a planet.
A close look at the above image shows something quite remarkable on the right hand side: 90377 Sedna. It can be seen quite clearly that despite that image covering about twice the width of Neptune’s orbit top to bottom, a distance it would take light almost a day to cross, only a small section of Sedna’s orbit is visible.
Sedna’s real orbit dwarfs the rest of the known Solar System:
Sedna takes so long to orbit the Sun that the last time it was in the current position the last Ice Age had yet to end, agriculture hadn’t been invented and there were still mammoths. It takes 114 centuries to go round. Compared to Sedna, Pluto is practically co-habiting. Given that Sedna is unusually close to the Sun right now, there’s a further implication: Sedna is very probably itself only one of a hundred or so worlds around the same size, orbiting way out in the depths of the Solar System in a region referred to as the Oort Cloud. There’s a chance, of course, that it’s the only such object but if that’s so, it’s quite a coincidence that it just happens to be situated where it is right now.
Sedna is bright red, almost as red as Mars in fact, like many other objects that far out are likely to be. Its surface is rich in tholins, alcohol-like substances with sulphur instead of oxygen. Tholins can’t form naturally in the inner solar system, so any body high in them is likely to have spent a lot of time extremely far from any stars. It also has methane and nitrogen on its surface, and is thought to be in the middle of a very brief two century long summer when some of its surface becomes gaseous and forms a temporary thin atmosphere. And yes, there is water ice on its surface but it’s ice but not as we know it. Speaking of which, it probably is heated internally by radioactivity, like many other worlds, meaning that it could actually have a water ocean inside, so it’s possible that even this place has life!
It has no known moons, and its surface is probably quite smooth owing to the fact that it spends most of its time so far out from the Sun that it rarely encounters anything else at all, so there are probably only very few craters on its surface if any, even early in the history of the planets when the inner solar system was relatively full of rocks hurtling about. Sedna is so far out that a few of the closer stars to the Sun, such as Alpha Centauri, would shift visibly to the naked eye from one side of the orbit to the other, although of course it would take the length of written human history to go that far. It’s also very cold, even though it’s currently the height of summer there right now, at about -261°C, cold enough to freeze hydrogen and meaning that there is only one possible non-frozen substance on its surface, namely helium, although since there’s nothing from which much of an atmosphere could form (apart from the occasional rogue molecule which just goes ahead and evaporates anyway), there wouldn’t be enough pressure for it to be liquid. Liquid helium is of course remarkable because it has a form which flows uphill and can trickle out of small leaks more easily than larger holes.
Consequently, Sedna is a pretty good new symbol for remoteness. However, even Sedna is lost in the vastness of the Universe compared to the nearest stars:
As can be seen from the caption, this shows its orbit from about four-fifths of a light year away, or about one-fifth of the way to Proxima Centauri. At some point between here and there, similar objects to Sedna will inevitably exist but orbiting other stars, and they can also be far enough out that there’s a chance that other stars will disturb their orbits, though only quite a small one (about one percent over the history of our Solar System).
As well as all this, it’s still quite feasible that a very distant planet remains to be discovered orbiting the Sun. If it exists, its year would last around fifteen millenia, it would be around ten times the mass of the Earth and two to three times its diameter. Its orbit would also be quite elliptical, like that of Sedna. The reason this may exist is that the orbits of objects out that far do still seem to be disturbed by something, and there’s a model of the early Solar System which requires an extra Neptune-like planet which was however thrown out of the visible part of the system early in its history. If such a planet does exist, it could replace Sedna as a sign of remoteness, but right now, Sedna is surely enough.