The seventh planet in terms of proximity to the Sun. It is the third largest planet in the solar system by radius and fourth by mass. The composition of Uranus is the same as that of Neptune, and both contain the same chemical elements, which differentiate them from the large gas giants Jupiter and Saturn. For this reason, scientists classify Uranus and Neptune as "ice giants" to distinguish them from gas giants. Although Uranus' atmosphere is similar to that of Saturn and Jupiter, with a composition of mostly hydrogen and helium, it contains more ice along with ammonia, methane, and other hydrocarbons.