Uranus is the seventh
planet from the
Sun; it is the third largest and fourth most massive planet in the
solar system. Uranus was the first planet discovered in
modern times. Though it is visible to the naked eye like the five
classical planets, it was never recognised as a planet by ancient observers due to its dimness. Sir
William Herschel announced its discovery on March 13, 1781, expanding the known boundaries of the solar system. Uranus' atmosphere, although similar to
Jupiter and
Saturn in being composed primarily of
hydrogen and
helium, contains a higher proportion of "ices" such as
water,
ammonia and
methane, along with the usual traces of
hydrocarbons. It has the coldest planetary atmosphere in the solar system, with a minimum temperature of 49
K, and has a complex layered
cloud structure in which water is thought to make up the lowest clouds, while methane makes up the uppermost layer of clouds. In 1986, images from the
Voyager 2 space probe showed Uranus as a virtually featureless planet in visible light without the cloud bands or
storms associated with the other giants. The
wind speeds on Uranus can reach 250 m/s (560 mph).
