Neptune from Triton. Illustration of a view of Neptune (upper centre) from the surface of its largest moon Triton. Neptune, the eighth planet from the Sun, has a diameter of over 48,000 kilometres. It is considered an ice giant, with an icy interior and an atmosphere of hydrogen and helium. Triton, its largest moon, consists of ices and rocks. It has a diameter of around 2700 kilometres, and orbits Neptune at a distance of around 355,000 kilometres.