Why is Pluto no longer considered a planet? 

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In 2006, the International Astronomical Union (IAU) declared that Pluto was no longer considered a planet but, instead, a dwarf planet. After Pluto was discovered in 1930, debate continued about whether it was a planet or maybe simply a small icy body lying beyond Neptune in an area that became known as the Kuiper Belt. In 1992, the first member of this area was identified. More Kuiper Belt Objects (KBOs) were discovered, including Quaoar (2002), Sedna (2003), and Eris (2005), which had roughly the same mass as Pluto.

These discoveries pushed the IAU to consider during their 2006 General Assembly in Prague what defined a planet. They produced a three-part definition. A planet should orbit around the sun. In addition, they declared that a planet has "sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape." The third part of the definition was that a planet has to "clear the neighborhood around its orbit." This means that it has to be dominant in terms of gravitation and have thrown other large objects out of its orbit. While Pluto met the first two criteria, it did not meet the last one, as there were other KBOs in the neighborhood of its orbit. Therefore, it was deemed not a planet. 

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