Neptune is a gaseous planet of the Solar System. Orbiting at 2.8 billion miles away from the Sun, it is the eighth and last planet of the system. It takes Neptune 164.8 years to complete an orbit around the Sun. It has a diameter of 30.775 miles and more than 17 times the Earth's mass. Neptune is composed mainly of ammonia, methane, and ices, and has a weak magnetic field. This Solar System planet has thirteen moons and a ring of dust-size particles.