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Uranus is the seventh planet from the Sun. It is named after Greek sky deity Uranus (Caelus), who in Greek mythology is the father of Cronus (Saturn), a grandfather of Zeus (Jupiter) and great-grandfather of Ares (Mars). Uranus has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. The planet is similar in composition to Neptune, and both have bulk chemical compositions which differ from those of the other two giant planets, Jupiter and Saturn (the gas giants). For this reason, scientists often distinguish Uranus and Neptune as "ice giants".
As with gas giants, ice giants lack a well-defined solid surface. Uranus's atmosphere is similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It has the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 kelvins (−224 °C; −371 °F). It has a complex, layered cloud structure; water is thought to make up the lowest clouds and methane the uppermost layer. The planet's interior is mainly composed of ices and rock. (Full article...)