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Written and fact-checked by
The Editors of Encyclopaedia Britannica
There are eight planets in the solar system. The four inner terrestrial planets are Mercury, Venus, Earth, and Mars, all of which consist mainly of rock. The four outer planets are Jupiter, Saturn, Neptune, and Uranus, giant planets that consist mainly of either gases or ice. Pluto was considered the ninth planet until 2006, when the International Astronomical Union voted to classify Pluto as a dwarf planet instead.
