Pluto: What Does It Take to Rate Planethood? When Pluto was first discovered in 1930, it was accepted as the ninth planet in our solar system. However, it lost that status in 2006 when the International Astronomical Union created a new classification of celestial bodies: dwarf planets. Dwarf planets, the category into which Pluto was moved, are similar to regular planets in two ways: both types of planets orbit the sun and contain sufficient mass to have been shaped into spheres by gravity. The primary difference between regular planets and dwarf planets involves their orbits. Regular planets have cleared their "neighborhoods" of other objects by deflecting or consuming the smaller bodies that they have encountered, leaving their orbits relatively unobstructed. A dwarf planet like Pluto, however, has not cleared its orbit. Pluto is one of many small, icy objects in its region of space, and since it does not have significantly more mass than these objects, it has not managed to absorb them, nor to move them away through direct contact or gravitational interactions.
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