Animation of Jupiter and the Galilean moons. Jupiter is the largest planet in the Solar System, more than eleven times the diameter of Earth, and 318 times Earth磗 mass. Indeed, its mass is 2.5 times the mass of the rest of the Solar System combined. It is a gas giant, composed largely of hydrogen, with helium and only traces of other gases. Its four largest moons were discovered by Galileo in 1610. They are, in order of appearance and distance from Jupiter: Io, Europa, Ganymede and Callisto. Ganymede is the largest moon in the solar system. The orbital paths of the four moons around Jupiter are seen towards the end of the animation. The orbital speeds and distances are to scale, Jupiter and the moons have been enlarged x2 and x10 respectively for clarity.