Saturn from Iapetus. Artwork of Saturn as seen from Iapetus, its third largest moon. Iapetus is primarily composed of water ice. Saturn is the sixth planet from the Sun and the second largest in the solar system. It is a gas giant, composed mostly of hydrogen, with small proportions of helium and trace elements. It has a prominent ring system, consisting mainly of ice and dust particles, and is orbited by at least sixty moons.