The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were discovered by Galileo Galilei around January 1610 and were the first group of objects found to orbit another planet. Their names derive from the lovers of Zeus. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other.