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What has NASA defined as a 10th planet?

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Quick Answer

In 2005, NASA announced that scientist Mike Brown discovered the dwarf planet now known as Eris, the 10th planet, in the Kuiper Belt beyond Neptune. While Eris is smaller than Earth's moon, it is larger than an asteroid and is therefore considered a dwarf planet.

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Full Answer

The planet was first noticed in 2003, but because of its slow movement, was not identified as a planet until the area was re-examined in 2005. Initially Eris, named after the Greek goddess of conflict, was nicknamed Xena. Eris' discovery triggered scientific discussions about the fundamental definition of a planet. While Eris is considered the 10th planet, Eris, Pluto, and the asteroid Ceres are all considered dwarf planets. Eris has a small moon named Dysnomia.

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