This picture was snapped by the Voyager 2 spacecraft in 1986 - the only spacecraft ever to visit Uranus. Uranus is the third largest planet after Jupiter and Saturn. Uranus has many moons and a ring system. Uranus is composed mostly of liquid water, methane and ammonia, surrounded by a thick gas atmosphere of mostly hydrogen and helium. Uranus is peculiar in that its rotation axis is greatly tilted and sometimes points near the sun. It remains an astronomical mystery why Uranus' axis is so tilted. Uranus and Neptune are very similar.