The largest moon in the solar system is Ganymede, which orbits the planet Jupiter. Ganymede is even larger than the planet Mercury. It has a diameter of about 5,268 kilometers (3,273 miles) and is the only moon in the solar system known to have its own magnetic field. In addition to Ganymede, Jupiter has three other large moons called the Galilean moons: Io, Europa, and Callisto.