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The seventh planet from the Sun and the third largest, discovered by William Herschel
in 1781. It has a diameter about 51,100 km, four times that of the Earth, and a mass
14.5 times Earth's mass. Uranus orbits the Sun at a distance over 19 times the Earth-Sun
distance. Although it takes about 84 years for Uranus to make a revolution, it completes
a fast rotation in only 17 and a half hours. Unlike the other planets, its axis of
rotation lies mostly in the plane of the Solar System. Uranus is internally less active
than the other giant planets, which added to its larger distance from the Sun, makes
it colder. It has a dense atmosphere made of mostly molecular hydrogen (83 percent)
and helium (15 percent), with two percent methane and traces of acetylene and other
hydrocarbons. The planet's greenish-blue color is due to light scattering as in Earth's
sky and the absorption of red light by its small amount of atmospheric methane. Uranus
has a ring system and 27 known satellites.
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