Neptune and Triton. Artwork of the planet Neptune (blue), seen with its moon Triton. Neptune is a gas giant planet, mainly composed of gaseous hydrogen and helium. Traces of methane in its upper atmosphere give it its blue colour. It orbits the Sun at an average distance of 4497 million kilometres. Neptune has a thin ring system, part of which is seen here. Triton is the largest of Neptune's eight satellites. It is the coldest world in the solar system, with a surface temperature of -235 degrees Celsius.