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By definition, a planet is a celestial body that is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes an almost circular orbit, and has cleared the neighborhood around its orbit.

That being said, the Solar System contains only 8 planets, and this has been so since 2006, when the above definition was created. However, if the dwarf planets, which are the bodies outside of Neptune (like Pluto), were to be named planets, the dwarf planet Eris would have most likely taken the title of the 10th planet.