A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite of a planet.
The term dwarf planet was adopted in 2006 by the International Astronomical Union, or IAU, to define the status of planets, including newly discovered bodies orbiting the Sun at vast distances past Pluto. Rather than have an open-ended number of planets, this classification set the number at 8, reclassifying Pluto as a dwarf planet. It shares this class with the asteroid Ceres and the Kuiper Belt objects Haumea, Makemake, and Eris. The planetoid Sedna is a current candidate for this classification, as are more than 10 other trans-Plutonian objects and the asteroids Vesta and Pallas.
In order to be classified as a planet, the object must meet three criteria:
- It must orbit the Sun, not another planet.
- It must have achieved "hydrostatic equilibrium", meaning that its gravity has crushed the object into a spherical shape.
- It must have "cleared the neighborhood" of its orbit, so that nothing else orbits the Sun near the planet.
The criteria were somewhat controversial, and are subject to revision by later conventions of the IAU. Objects that met the first two criteria but not the third were classified as "dwarf planets".
Interestingly enough, the planet Earth also fails to meet the third criterion, as there are a few small asteroids which occasionally approach the Earth in their own orbits around the Sun.