The sixth planet from the sun is Saturn. It is also the second largest planet in the solar system. The most distinctive feature is its ring system, which was first seen in 1610 by Galileo. These rings are now known to be made of more than 100,000 individual ringlets. The rings are thought to consist of rock, frozen gases, and ice, and range from dust to boulder size. Saturn radiates three times as much heat into space as it receives from the sun. Saturn's atmosphere is mostly made up of hydrogen and helium. More than 20 satellites have been discovered orbiting Saturn. Saturn's largest moon, Titan, is larger than Mercury.