• The sixth planet of the solar system and its largest apart from Jupiter. Saturn has an equatorial diameter of 120,000km, but its polar diameter is 10 per cent less, giving it much the greatest polar flattening of any planet of the solar system. Saturn is also the least dense of the planets, at about 0.7gm/cc. Saturn is accompanied by its famous ring system and by a large family of satellites, and is 95 times as massive as the Earth. Its globe has a system of cloud belts and zones like those of Jupiter but on a smaller scale and displaying more gradual change. Saturn rotates on its axis in 10 hours and has a 29-year orbit at an average of 1427 million km from the Sun. Most of our knowledge of Saturn and its satellites comes from brief visits by the US Pioneer 11 and Voyager 2 spacecraft. At its immense distance from the Sun, the ‘surface’ temperature at the cloud tops is only about 95K, but the observed cloud zone takes up less than 1 per cent of the planet’s radius. Beneath the clouds is thought to lie a deep layer of gas, mainly hydrogen and helium, which lower still turns into solid metallic hydrogen. At the planet’s very heart there is likely to be a core of rocky and/or icy material. Both this and the metallic hydrogen layer are likely to be much smaller than their equivalents within Jupiter, since Saturn has less mass and hence less of the internal pressure needed to form these zones. Its lower overall density also limits their possible sizes. The outer cloud layers of Saturn contain methane and ammonia as well as hydrogen and helium. Saturn has nine major satellites: Mimas, Enceladus, Tethys, Dione, Rhea, Titan (itself bigger than Mercury), Hyperion, Iapetus and Phoebe and a large number of smaller satellites, some of which dictate the gaps in Saturn’s ring system. First identified correctly by Huygens, smaller versions of the ring system have since been found at other gas giant planets, but none are as large or as spectacular as Saturn’s. The rings of Saturn may have formed by the breakup of a large astroid-like object within the last few million years.