The Kuiper Belt is not the outer limits of our solar system, though. Beyond it there are more objects, collectively referred to as the Scattered Disk. These include (probably) Sedna, which was discovered in 2003, and Eris, discovered in 2005, which is heavier than Pluto and has a moon of its own. It was the discovery of Eris which led directly to the demotion of Pluto. Eris orbits the sun at between about 40 and 100 AU; Sedna between about 80 and 900 AU (it has a very elongated orbit).
Based on the behaviour of these Kuiper Belt and Scattered Disk objects, astronomers have now come out again in favour of the theory that there is a ninth planet in our solar system (see this news article, which includes a diagram showing the basis for the theory).
The proposed planet is a small ice giant (if that’s not a paradox) like Uranus or Neptune, maybe 10 times as heavy as Earth, orbiting the Sun in an eccentric orbit between 200 and 1200 AU. Based on its size and the fact that it has almost certainly cleared its neighbourhood, there would be no doubt it’s a planet. Telescopes have begun searching for it. If it is near the aphelion of its orbit (i.e. far from the Sun) it could be very hard to find.
What I find enjoyable about the whole thing is that it comes across a bit like an Agatha Christie novel:
- The Kuiper Belt terminates suddenly at about 48 AU – why? Is there something big out there clearing the neighbourhood?
- What dragged Sedna and the other sednoids into their current weird orbits?
- Given that we only know about the Scattered Disk objects that are close to the perihelion of their orbits, what does probability lead us to expect is currently at their aphelion, and where are they? Or, per astronomer Mike Brown: “Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto [out there] then there are probably forty objects the size of Pluto ... If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, and the biggest of these objects ... is probably the size of Mars or the size of the Earth.”