Prior to the discovery of Eris, the largest currently known object beyond the orbit of Neptune, the status of Pluto was debatable, due to its small size compared to even the smallest of the other major planets. Once it was found that Eris was about the same size as or perhaps even a little larger than Pluto, the debate intensified, until -- in August 2006 -- the International Astronomical Union decided to demote Pluto to "dwarf planet" status (the vote was about 4% for and 3% against, with 93% of members abstaining because they felt the debate was a waste of their time). Since what Pluto is called makes no difference to what it is, the debate and its results were probably pointless, but some people like to stick things into neat little boxes, no matter how poorly they fit.
Having decided to demote Pluto the question arose as to what objects such as Pluto should be called. It had been realized for a long time that Pluto was neither a Terrestrial planet (a small, rocky and metallic object close to the Sun), nor a Jovian planet (a large ball of liquified gases and ices, far from the Sun), but simply one of the largest icy bodies orbiting the Sun. In August 2008 it was decided that such objects should be called Plutoids (basically, something similar to Pluto). The diagram above shows the largest known Plutoids and their currently assigned names and where appropriate the names of their known moons (natural satellites).
There is also a group of objects known as Plutinos. These are small icy bodies similar to Plutoids, but are distinguished from them by the way they move around the Sun. Neptune was originally formed a little closer to the Sun than now, and as it swept up material left over from the formation of the Solar System, gradually moved outwards. As it did so it began to interfere with the orbital motions of a number of objects, including Pluto, in such a way that their orbits became locked to the orbit of Neptune. In Pluto's case there is a ratio of 3 to 2 for the length of its orbital period compared to that of Neptune. For other Plutinos the ratio is 3 to 2 or 4 to 3, or even 2 to 1; but in every case the orbital periods are in the ratio of two small whole numbers. This is an example of commensurability
, which is also discussed under Orbital Regularities
. All Plutinos are probably Plutoids, but only those Plutoids with orbital periods commensurate with that of Neptune are Plutinos.
All Plutoids and Plutinos are also KBO's, meaning Kuiper Belt Objects, because they lie within a region extending well beyond the orbit of Neptune, into which immense numbers of icy bodies, large and small, were thrown by the Jovian planets as they swept the region between themselves clear of smaller objects, more than four billion years ago. The Kuiper Belt is actually a disk-shaped region a thousand or so Astronomical Units in size, centered on the Sun; but from inside the Solar System, objects in the disk are always seen in a band centered on the Ecliptic, extending all around the sky -- hence the name Kuiper "Belt" Objects.