Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Among the giant planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune. Neptune orbits the Sun at an average distance of 30.1 astronomical units (4.50×109 km).