Pluto, Eris, and the other dwarf planets are half as big as the planets in diameter and 10 times smaller in mass. Unofficially, though, size is a part of the case against Pluto as well. Rather than add all these to the list of planets, the IAU decided to create the new category of dwarf planet. ( NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker)īecause astronomers discovered Pluto decades before any of the other Kuiper belt objects (KBOs), they long assumed it was one of a kind and called it a planet.īut since 1992, they've discovered more than a thousand KBOs, including Eris, a dwarf planet that's just a bit smaller in diameter than Pluto and about 27 percent more massive. Pluto and other Kuiper belt objects (note: not shown to scale). Pluto meets the first two requirements, but not the third, because it's part of a cloud of similarly sized debris that orbits the sun beyond Neptune: the Kuiper belt. According to the IAU, for an object to be a planet it needs to meet three criteria: It has to orbit the sun, be large enough that the force of its gravity pulls it into a spherical shape, and have "cleared the neighborhood around its orbit." In theory, an object that's large enough will clear its orbit either by collecting other debris with its gravity or by surviving impacts with it.
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