We tend to picture the Moon as something every planet should have one of. But Jupiter has been collecting them. Astronomers have now confirmed more than 95 separate moons orbiting Jupiter - most are tiny, but four are huge.
Those four - Io, Europa, Ganymede, and Callisto - are called the Galilean moons, because they were the first moons ever discovered orbiting another planet. Galileo Galilei spotted them in 1610, just by pointing his early telescope at Jupiter. Each one turned out to be a small world of its own.
Ganymede is the biggest. It’s bigger than the planet Mercury, and bigger than Pluto. If it were orbiting the Sun on its own, it would easily count as a planet. Europa has an ice-covered ocean of liquid water underneath, which might be one of the best places to look for alien life in our solar system. Io is the most volcanically active body in the solar system, with eruptions visible from space.
