Saturn

Saturn is the sixth planet from the Sun and the second-largest in the Solar System, another hydrogen-helium gas giant. Its defining feature is its vast ring system — by far the most extensive in the Solar System — composed of trillions of chunks of almost-pure water ice ranging from micrometer-sized dust to house-sized boulders, organized into distinct named rings separated by narrow gaps shaped by small shepherd moons.

Saturn has the lowest density of any planet in the Solar System (0.687 g/cm³), less than water. Its atmosphere shows subtler banding than Jupiter’s but hosts exotic features such as the hexagonal north-polar vortex — a stable, six-sided jet-stream pattern ~14,500 km across.

Saturn’s 146 known moons include Titan — the only moon with a substantial atmosphere and the only world other than Earth where liquid rivers, lakes and seas have been observed on the surface (methane and ethane, not water). Enceladus, a small icy moon, erupts plumes of water vapor and ice grains from a subsurface ocean in contact with rocky seafloor — making it a prime target in the search for life. The Cassini-Huygens mission (2004-2017) transformed our understanding of the entire Saturnian system before Cassini’s planned atmospheric entry in September 2017.