Neptune (lower left) and its moon Triton (upper right). Neptune is the eighth planet from the Sun, orbiting at a distance of over 4.5 billion kilometres. It has a diameter of over 48,000 kilometres, around 3.8 times that of Earth, and a volume of around 55 Earths. 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. Image obtained on 19 September 2015.