The Galilean moons are four large moons of the planet Jupiter: Io, Europa, Ganymede, and Callisto. They were discovered by the astronomer Galileo Galilei in 1610 and are among the largest moons in the solar system. Each moon has unique characteristics, such as Io's volcanic activity and Europa's icy surface. These moons are significant for scientific study because they may provide insights into planetary formation and the potential for extraterrestrial life. For example, Europa is believed to have a subsurface ocean beneath its ice, making it a target for future exploration by space missions.