Neptune

Neptune (pronounced /'n?ptju? n/[8]) is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third major by mass. Neptune is 17 times the mass of Earth and is somewhat more massive than its near-twin Uranus, which is 15 Earth masses and less dense. The planet is named following the Roman god of the sea. Its astronomical symbol is a stylized story of Poseidon's trident.

Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction quite than regular observation. Unexpected changes in the orbit of Uranus led astronomers to realize the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predict position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered preceding to the twentieth century. Neptune has been visit by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.