Neptune Neptune is the outermost planet of the gas giants. It has an equatorial diameter of 49,500 kilometers (30,760 miles) and is the eighth planet from the sun. If Neptune were hollow, it could contain nearly 60 Earth's. Neptune orbits the Sun every 165 years. It has eight moons, six of which were found by Voyager 2. A day on Neptune is 16 hours and 6.7 minutes. Neptune was discovered on September 23, 1846 by Johann Gottfried Galle, of the Berlin Observatory. Neptune got its named from the Roman God of the Sea. Much of what is know today about Neptune was discovered in 1989 by the U.S Voyager 2 spacecraft during