Illustration showing the planet Jupiter (right) with Io, one of its moons (left). Jupiter is the fifth planet from the Sun and the largest in the solar system, with a mass two and a half times that of all the other planets put together. Io is the innermost of the four Galilean moons of Jupiter and has a diameter of 3,642 kilometres, making it the fourth-largest moon in the Solar System. It is also the most volcanically active object in the solar system, with hundreds of volcanoes, some of which erupt lava fountains dozens of kilometres high.