Neptune and Triton. Illustration of the gas giant planet Neptune showing its faint rings, as well as its largest natural satellite, the icy moon Triton. Neptune mainly comprises gaseous hydrogen and helium, with small amounts of methane. It has a diameter of around 50,000 kilometres, almost four times that of the Earth. It orbits the Sun at a distance of 4500 million kilometres, taking almost 165 years for each orbit. Its blue colour is due to methane in its upper atmosphere. The Great Dark is thought to be a large storm system, and it has numerous white, high-altitude clouds associated with it. Triton is famous for its size, almost as large as Earth??? own moon, and for its frigid geysers and odd terrains.