dwarf planet, a nonluminous body of rock or gas that orbits the sun and has a rounded shape due to its gravity. Unlike a planet, a dwarf planet is not capable of clearing its orbit of smaller objects by collision, capture, or other means. The known dwarf planets in the solar system include Ceres, Pluto, and Eris. All of these are significantly smaller in size and mass than Mercury. Eris, which is slightly larger than Pluto, is the largest known dwarf planet. Except for Ceres, all known dwarf planets follow orbits that lie or take them beyond Neptune into the outer reaches of the solar system.
Ceres and Pluto have been considered planets at time in the past. The discovery of other bodies in the asteroid belt led to the reclassification of Ceres in the mid-19th cent. as an asteroid. The discovery of other bodies beyond the orbit of Neptune that were similar in size to Pluto led to the declassification of Pluto as a planet in 2006, when the International Astronomical Union adopted the classification of dwarf planet.
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