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By the IAU definition, a planet must have "cleared the neighbourhood around its orbit". Pluto was demoted from planet to dwarf planet because it does not satisfy that condition. But with Pluto's orbital path crossing that of Neptune, couldn't it be said that Neptune has failed that qualification as well? Shouldn't Neptune be considered a dwarf planet?

Neptune is still considered a planet because its mass is much greater than the combined mass of everything else that crosses its orbit (including Pluto). Neptune is about 8000 times more massive than Pluto, so Neptune is a planet and Pluto is a dwarf planet.

About the Author

Sean Marshall

Sean is a sixth-year astronomy graduate student at Cornell who works with Professor Donald Campbell to study physical properties of near-Earth asteroids using radar and infrared data. Sean currently manages the "Ask an Astronomer" website.

 Website:  http://astro.cornell.edu/~seanm/

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