Ganymede (Jupiter III) is the largest moon of Jupiter and in the Solar System, and the only moon known to have a magnetosphere. It is the seventh satellite outward from Jupiter and third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. Ganymede has a diameter of , 8% larger than the planet Mercury, but its mass is only 45% that of Mercury. Ganymede is 2% larger than Saturn's Titan, the Solar System's second-largest moon. At 2.02 times the mass of the Moon, it is the most massive planetary satellite. It is the ninth-largest object in the Solar System, and the largest without a substantial atmosphere.