Saturn
Saturn is the sixth planet from the Sun and the second largest in the solar system, a gas giant made mostly of hydrogen and helium with the lowest mean density of any planet — 0.69 grams per cubic centimetre, less than that of water. Its rings, almost entirely water ice, extend up to 282,000 kilometres from the planet yet stand only about 10 metres thick in the main rings, and its family of 293 confirmed moons as of August 2026 ranges from planet-sized Titan, with its thick nitrogen atmosphere and methane lakes, to Enceladus, whose geysers spray a subsurface ocean into space and supply the E ring. NASA's Cassini spacecraft orbited Saturn from 2004 to 2017 — the first spacecraft ever to do so — and its ESA-built Huygens probe made the first landing in the outer solar system, on Titan, in January 2005.