Like other outer planets in the solar systm, Neptune is a gas giant, about 3.5 times the diameter of the Earth with a thick hydrogen atmosphere surrounding a small rocky core. Small amounts of methane in the atmosphere make the planet appear blue as it absorbs red light. Although not as prominent as on Jupiter and Saturn, Neptune has active weather systems with strong winds and storms. This virtually tue-colour image, taken by the Voyager 2 spacecraft in 1989, shows the Great Dark Spot (left), a giant storm system that has since disappeared.