Uranusis the seventhplanetfrom theSun. It has the third-largest planetary radius and fourth-largest planetary mass in theSolar System. Uranus is similar in composition toNeptune, and both have different bulk chemical composition from that of the largergas giantsJupiterandSaturn. For this reason, scientists often classify Uranus and Neptune as "ice giants" to distinguish them from the gas giants. Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition ofhydrogenandhelium, contains more "ices", such as water,ammonia, andmethane, along with traces of otherhydrocarbons.It is the coldest planetaryatmospherein the Solar System, with a minimum temperature of 49 K (−224.2 °C), and has a complex, layeredcloudstructure, with water thought to make up the lowest clouds, and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock.