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Solar system planets. Computer artwork of the eight planets of the solar system, which are arrayed from left to right in order of distance from the Sun (left). The four small rocky planets of the inner solar system are Mercury, Venus, Earth and Mars. The four large gas giant planets of the outer solar system are Jupiter, Saturn, Uranus and Neptune. Pluto was re-categorised as a Dwarf planet in 2006, so it is not included here. At lower right is a galaxy, and a small celestial body (an asteroid or comet) is seen at lower left. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA/JPL/SCIENCE PHOTO LIBRARY)