Body, other than a natural satellite (moon), that orbits the Sun and that is, for practical purposes, smaller than the planet Mercury yet large enough for its own gravity to have rounded its shape substantially. The International Astronomical Union adopted this category of solar system bodies in August 2006, designating Pluto, the even more-remote object Eris, and the asteroid Ceres as the first members of the category. Unlike major planets, these three bodies are not massive enough and are in orbits too elliptical, too inclined, or both to have swept up most smaller nearby bodies by gravitational attraction; they thus failed to grow larger.
This entry comes from Encyclopædia Britannica Concise.
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