Animation showing the relative sizes of the Earth and Neptune. Earth has a diameter of some 12,742 kilometres, making it the largest of the rocky terrestrial planets. Neptune is the smallest of the giant planets, just under four times Earth磗 width, with a diameter of some 49,500 km. Neptune has a thin ring system composed of dark particles. It orbits the Sun at a distance of some 4.5 billion kilometres, thirty times the Earth_Sun distance.