Almost two years after the International Astronomical Union (IAU) caused a worldwide furore by stripping Pluto of its former status as a 'proper' planet to a dwarf planet, the term 'plutoid' has been introduced to describe "Pluto-like transneptunian dwarf planets".
The IAU's new Solar System, as defined in 2006, with Pluto, Eris and Ceres named as dwarf planets. Now, Pluto and Eris are plutoids, while Ceres remains a dwarf planet. Image: IAU.
The original demotion of Pluto to a dwarf planet came about as a result of numerous discoveries of Pluto-like bodies, some even larger in size, in the far reaches of our Solar System. If they were treated the same as Pluto, they too would have to be called planets, taking the Solar System’s planet inventory to more than 50, a prospect that was even less favourable than relegating just one planet to a sub-category, which also included the bodies Ceres and Eris.
Now, the IAU have once again re-written the textbooks to introduce a new term – 'plutoid' – to describe “celestial bodies in orbit around the Sun at a distance greater than that of Neptune, that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a near-spherical shape, and that have not cleared the neighbourhood around their orbit of debris”. The body must also have an absolute magnitude brighter than +1 to be considered as a plutoid and be named by the IAU as one. If, subsequently, the plutoid candidate turns out to not be massive enough to be classified as one, it will still keep its name, but will change category.
The new plutoid category of Solar System bodies includes Pluto and its moons Charon, Hydra and Nix (left) and Eris and its moon Dysnomia (right). Image: IAU.
The new classification systems means that while Pluto and Eris are the first plutoids of the Solar System, Ceres remains a dwarf planet, because it is located in the asteroid belt between Mars and Jupiter. It is expected that more plutoids will be named as science progresses and new discoveries are made.