- 1 Roman Mythology the god of water and of the sea. Greek equivalent Poseidon.
Neptune orbits between Uranus and Pluto at an average distance of 4,497 million km from the sun. It is the fourth-largest planet, with an equatorial diameter of 49,500 km, and the most remote of the gas giants. The planet is predominantly blue, with an upper atmosphere mainly of hydrogen and helium with some methane. There are at least eight satellites, the largest of which is Triton, and a faint ring system