Saturn, second-largest planet in the Solar System, the sixth from the sun. Until the discovery of Uranus (1781), Saturn was the outermost planet known. It orbits the sun in 29.46 years at a mean distance of 886.7 million mi (1.427 billion km). Saturn has the lowest density of any planet in the Solar System, less than that of water, and may contain over 60% hydrogen by mass. Its total mass is about 95 times that of the Earth. Saturn has 17 known satellites; the largest, Titan, about the same size as Mercury, has a cold nitrogen atmosphere with traces of methane and other gases. Other major satellites include the Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, Phoebe, and Hyperion. The most striking feature of Saturn is its ring system, composed of countless tiny particles of ice and rock. Three or four major ring divisions are visible from Earth; space probes (1980, 1981) revealed the rings to consist of hundreds of narrow ringlets. The rings are about 10 mi (16 km) thick.