The International Astronomical Union (IAU) has awarded poor old Pluto a consolation prize following its controversial demotion from the league of planets - other similar dwarf planets will henceforth be called "plutoids".

Pluto was given its marching orders back in 2006 in an IAU resolution which clarified:

(1) A "planet" [1] is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

Pluto, the IAU concluded, was a "dwarf planet by the above definition and is recognized as the prototype of a new category of trans-Neptunian objects".

Well, these trans-Neptunians are now called Plutoids, the IAU executive committee has announced. According to Reuters, the group currently comprises just Pluto and Eris, which is actually bigger than the former planet, although astronomers "expect to find more".

The biggest known asteroid Ceres, meanwhile, doesn't qualify for membership of the Plutoid club since lies in the asteroid belt between Mars and Jupiter. ®

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