MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Dwarf planets. Computer artwork of three dwarf planets next to the Earth (bottom), showing their relative sizes. From left: Eris (formerly 2003 UB313), Pluto, and Ceres. A dwarf planet is defined as an object large enough to be spherical due to its gravity, and with an independent orbit around the Sun (ie. A true planet, such as the Earth, has to dominate its orbit around the Sun. Pluto and Eris fail because they are one of many Kuiper Belt objects in the outer solar system. Ceres fails because it is one of many objects in the asteroid belt.
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