Dwarf planet
A dwarf planet is a body that orbits the Sun and is round, but has not cleared its orbit of other objects.
In 2006 the International Astronomical Union defined a planet as a body that orbits the Sun, is massive enough to pull itself into a round shape, and has cleared its orbital neighbourhood of other material. A dwarf planet meets the first two rules but not the third.
Five bodies are officially recognised as dwarf planets: Ceres in the asteroid belt, and Pluto, Eris, Haumea and Makemake beyond Neptune. Several more, such as Gonggong, Quaoar and Sedna, may also qualify. Ceres is the only dwarf planet in the inner Solar System and is also the largest object in the asteroid belt.
The decision to reclassify Pluto was controversial, and some scientists still prefer a definition based on a body's own properties rather than its surroundings. Either way, dwarf planets are fascinating worlds, and NASA's Dawn mission to Ceres showed that one can have briny water below its surface.
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