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Solar System planets, illustration. Included are the 8 planets of the Solar System and their moons, the dwarf planet Pluto and its moon and the hypothetical ninth planet Planet X. From left to right, or from the Sun outwards, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and Planet X. (KEYSTONE/SCIENCE PHOTO LIBRARY/DETLEV VAN RAVENSWAAY/SCIENCE PHOTO LIBRARY)