Neptune is the eighth planet from the Sun in our Solar System. Named for the Roman god of the sea, it's the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth-Sun distance. Its astronomical symbol is . Discovered on September 23, 1846, Neptune was the fi… (More on Neptune)