What is a planet? The International Astronomical Union (IAU) said that the definition for a
planet is now officially known as "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 neighborhood around its
orbit." At the same time, new moons are also being discovered, both around existing planets
and within these mysterious new worlds. Once the existence of a moon is confirmed and its
orbit determined, the moon is given a final name by the International Astronomical Union
(IAU), the organization that assumed this task since 1919.
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, (c) has not cleared the neighborhood around its orbit, and
(d) is not a satellite.
Plutoids
Almost two years after the International Astronomical Union (IAU) General Assembly introduced
the category of dwarf planets, the IAU, as promised, has decided on a name for trans-neptunian
dwarf planets similar to Pluto. The name "plutoid" was proposed by the members of the IAU
Committee on Small Body Nomenclature (CSBN), accepted by the Board of Division III, by the IAU
Working Group for Planetary System Nomenclature (WGPSN) and approved by the IAU Executive
Committee at its recent meeting in Oslo, Norway.
Plutoids are celestial bodies in orbit around the Sun at a semi-major 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
neighborhood around their orbit. Satellites of plutoids are not plutoids themselves, even if they
are massive enough that their shape is dictated by self-gravity. The three known and named plutoids
are Pluto, Eris and as of July 2008, MakeMake. It is expected that more plutoids will be named as
science progresses and new discoveries are made. For more information about the IAU and plutoids,
click here.