Illustration depicting Jupiter's surface and three of its moons. Jupiter is the largest planet in the solar system with a diameter of around 140,000 kilometres - more than ten times that of Earth. As a gas giant, it is mostly comprised of hydrogen and helium, with swirling bands of colourful clouds made of ammonia, sulphur and phosphorus. The planet is particularly well known for the Great Red Spot (upper right), a massive storm at its centre that has been raging for hundreds of years. The largest three of Jupiter's many moons are shown here, Callisto, Europa and Ganymede, clockwise from left. These moons were first observed by the astronomer Galileo Galilei in 1610, using an early version of the telescope.