The Kuiper belt is strange.

Most of this strangeness probably comes from the fact that we are only just beginning to uncover this mysterious region of the Solar System. Unlike the Oort Cloud which (possibly) lies beyond 3 × 1012 km away (over 20,000 AU, or a whopping 0.3 light years), we can actually observe the objects inside the Kuiper belt as, compared to the Oort Cloud, the Kuiper belt is on our interplanetary doorstep.

But that doesn’t mean it’s close. The Kuiper belt exists in a region of space 30–55 AU from the Sun; this is where Pluto lives (as Pluto itself is a “Kuiper belt object”, or KBO). As astronomical techniques become more advanced however, we are able to discover more KBOs in the zoo of icy-rocky bodies that live in this region.

Pluto has had a rough ride lately. Firstly, in 2006, the International Astronomical Union (IAU) controversially kicked Pluto out of the Planetary Rotary Club, demoting it to a “dwarf planet”. The IAU had its reasons, but the world was shocked and upset at the decision to change its status.

Pluto was Plutoed.

While Pluto slowly pottered through the Kuiper belt on its 258 year orbit of the Sun, the argument raged here on Earth as to what Pluto actually was. Many wanted Pluto to be re-instated as a planet, but there was a bigger body out there, the trans-Neptunian object (TNO) Eris, that is bigger than Pluto. At first Eris was called the “tenth planet” when it was identified in 2005, but quickly astronomers realised the term “planet” might not apply to the increasing number of “minor planets” in the Kuiper belt and beyond. Unfortunately, Pluto was in the firing line and the IAU made its decision. However, all was not lost. This year, a new class of dwarf planet was defined, named after the demoted ninth planet: the “Plutoid”.

The IAU defines a Plutoid as:

…celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit. Satellites of plutoids are not plutoids themselves.

So is this the last we’ve heard of the “Pluto debate?” Probably not.

Although a whole class of objects have been designated as “Plutoids” (including Eris, Haumea, and Makemake) this will be of little comfort to Pluto. The dwarf planet may have an identity crisis, but it has another problem. It looks like Pluto’s two small moons, Nix and Hydra (orbiting at 48,700 kilometres and 64,800 kilometres from Pluto respectively), didn’t originate from the dwarf planet. Pluto’s kids are illegitimate (why do I feel Charon had something to do with this?).

Poor Pluto.

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