Glossary · Thiên văn học
Dwarf planet
By the IAU's 2006 definition: a body that orbits the Sun and has enough mass for its own gravity to pull it into a nearly round shape, BUT has not cleared the neighbourhood around its orbit, and is not a satellite.
The definition adopted by the IAU General Assembly in 2006, Resolution 5A, sets three conditions for a planet: it orbits the Sun, it has enough mass to reach hydrostatic equilibrium and so is nearly round, and it has cleared the neighbourhood around its orbit.
A dwarf planet meets the first two but NOT the third, with the added condition that it is not a satellite. Exactly one criterion, clearing the orbital neighbourhood, separates the two groups.
That criterion is what left the Solar System with exactly eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The first three members of the dwarf planet category were Ceres, Pluto and the body then known by the provisional designation 2003 UB313, later named Eris.