Neptune is the eighth planet from the sun, named after the Roman god of the sea. It is the fourth largest planet by diameter and third largest by mass, about 17 times the size of Earth. Neptune is composed primarily of water, methane and ammonia and has a distinctive blue color. It has 13 known moons, the largest being Triton. Neptune was discovered in 1846 and is located an average distance of about 2.8 billion miles from the sun.