DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY DETLEV VAN RAVENSWAAY / SCIENCE PHOTO LIBRARY
Earth and Moon with dwarf planets. Scaled computer artwork of (from left) Earth, the Moon, Pluto and Ceres. The latter are two of the solar system's dwarf planets. Pluto, which used to be a planet, was demoted in August 2006 to the status of dwarf planet, reducing the solar system's number of planets to eight. The definition of a dwarf planet is an object large enough to be spherical due to gravity, and with an independent orbit. A true planet, such as the Earth, has to dominate its orbit around the Sun. Pluto fails because it is one of many Kuiper Belt objects in the outer solar system. Ceres fails because it is one of many objects in the asteroid belt. The Moon fails because it is a satellite of the Earth.
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