Jupiter is the fifth planet from the Sun and is the largest planet in the solar system. If Jupiter were hollow, more than one thousand Earths could fit inside. It also contains two and a half times the mass of all the other planets combined. It has a mass of 1.9 x 1027 kg and is 142,800 kilometers (88,736 miles) across the equator. Jupiter possesses 62 known satellites. The four largest are Callisto, Europa, Ganymede and Io, and were named after Galileo Galilei who observed them as long ago as 1610. The German astronomer Simon Marius claimed to have seen the moons around the same time, but he did not publish his observations and so Galileo is given the credit for their discovery.