Neptune. Artwork of Neptune, the outermost planet in the solar system. Neptune is a gas giant, composed mostly of hydrogen and helium with some methane. It has a diameter of nearly four times that of the Earth. The planet's blue colour is thought to result from absorption of red light by the methane in its atmosphere. The Great Dark Spot (lower left) is thought to be a large storm system that has numerous white, high- altitude clouds associated with it.