The Galilean moons are four large satellites of the planet Jupiter. They were discovered by Galileo Galilei in the early 17th century, hence their name. The four moons are Io, Europa, Ganymede, and Callisto. They are among the largest objects in the solar system outside the sun and the eight planets, with Ganymede being the largest moon in the solar system, even bigger than the planet Mercury. These moons are very diverse, each offering unique geologies and potential for life. For instance, Io is volcanically active, while Europa and Ganymede are believed to harbor subsurface oceans. The Galilean moons have been the focus of numerous space exploration missions due to their intriguing features and potential habitability.