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Uranus. Computer artwork of Uranus, the seventh planet from the Sun. Uranus is a gas giant, composed mostly of hydrogen and helium with some methane. It has a diameter four times that of the Earth. The planet's blue colour is thought to result from absorption of red light by the methane in its atmosphere. (KEYSTONE/SCIENCE PHOTO LIBRARY/CHR 366 FRIEDRICH SAURER/SCIENCE PHOTO LIBRARY) === ===