Neptune is the eighth planet in the solar system with respect to its average distance from the Sun. It is the outermost of the giant gas planets (along with Jupiter, Saturn, and Uranus). The discovery of Neptune was one of the triumphs of mathematical astronomy. To understand the perturbations in the orbit of the planet Uranus, astronomers John Couch Adams and Urbain Jean Joseph Leverrier independently calculated the existence and position of the new planet. Then, Johann Gottfried Galle, of the Berlin Observatory, discovered Neptune on September 23, 1846, based on the theoretical mathematical calculations sent to him by Leverrier.

The diameter of Neptune is approximately 3.88 times the size of Earth's diameter, with an equatorial diameter of 30,763 mi (49,528 km) and a polar diameter of 30,236 mi (48,680 km). Its mass is about 17.2 times that of Earth's mass, with a value of 7.019 x 1024 slug (1.024 x 1026 kg). Neptune's volume is equivalent to about 60 earth-volumes...