Dwarf planet

Any member of one of the three major classes of objects belonging to the solar system (the other two classes being planets and small solar system bodies).

At the 2006 meeting of the International Astronomical Union (IAU), a dwarf planet was defined as a body that orbits the Sun (as opposed to any other body, such as a planet or satellite); that is massive enough to be pulled into a virtually spherical shape by its gravitational self-attraction; and that has not swept the space around itself clear of smaller debris.

Only three bodies were known to fit this definition at the time it was settled on: Ceres, the largest of the asteroids and the first to be discovered; Pluto, hitherto regarded as the ninth planet but controversially demoted as a result of the definition; and Eris, whose discovery in 2005 sparked the discussion that led to the definition of the dwarf planets.

However, many more undiscovered dwarf planets must exist in the outer regions of the solar system. Among known bodies that may be reclassified as dwarf planets in the future is Pluto's companion, Charon. If it is viewed as making up a twin planet with Pluto, it will share Pluto's dwarf-planet status; if it is viewed as a satellite of Pluto, it will not. The large asteroids Vesta, Pallas, and Hygieia may also be classified as dwarf planets.