Neptune

Neptune is the 8th planet from the Sun.

It is the 4th largest planet in the Solar System if we go by diameter. It is also the 3rd most massive planet and the densest giant planet. Neptune is 17 times more massive than the Earth. It is slightly more massive than Uranus. These two being known as the 'ice giants.' 

Neptune is composed mostly of gases and liquids. It has no well defined 'solid surface.' A year on Neptune is 146.8 Earth years. That's how long it takes for Neptune to go around the Sun. The distance from Neptune to the Sun is 30.1 AU. Remember and AU is the distance from the Earth to the Sun. 

Neptune is named for the Roman god of the sea, and the planet's astronomical symbol is based on its trident. 

Neptune was the only planet to be discovered in the Solar System based on mathematical prediction rather than observation. 

It was unexpected changes in the orbit of Uranus that led Bouvard to conclude that its orbit was being gravitationally perturbed by an unknown planet.