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 known today about Neptune was discovered in 1989 by the U.S Voyager 2 spacecraft during its 1989 flyby Neptune. 
Neptune as compared to Earth is 3.9 times the diameter, 30 times the distance from the sun, 17 times as massive, and 0.3 times the density.
Neptune travels around the Sun in an elliptical orbit at an average distance of 4.504 billion km (2.799 billion miles). Neptune consists largely of hydrogen and helium, and it has no apparent solid surface. The first two thirds of Neptune is composed of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a mixture of heated gases comprised of hydrogen, helium, water and methane. The atmospheric composition is 85% Hydrogen, 13% Helium, and 2% methane. The planet’s atmosphere, particularly the outer layers, contains substantial amounts of methane gas. Absorption of red light by the atmospheric methane is responsible for Neptune’s deep blue color.
Neptune is a dynamic planet with several large, dark spots reminiscent of Jupiter’s hurricane-like storms. The largest spot, known as the Great Dark Spot, is about the size of the earth and is similar to the Great Red Spot on Jupiter.
Neptune receives less than half as much sunlight as Uranus, but heat escaping from its interior makes Neptune slightly warmer than Uranus. The heat liberated may also be responsible for Neptune’s stormier atmosphere, which exhibits the fastest winds seen on any planet in the solar system. Most of the winds there blow westward, opposite to the rotation of the planet. Near the Great Dark Spot, winds blow up to 2,000 kilometers (1,200 miles) an hour. Voyager 2 found that the winds averaged about 300 meters per second (700 miles/hour) in the planet’s atmosphere.
Long bright clouds, similar to cirrus clouds on Earth, were seen high in Neptune’s atmosphere. At low northern latitudes, Voyager captured images of cloud streaks casting their shadows on cloud decks below.
Feathery white clouds fill the boundary between the dark and light blue regions on the Great Dark Spot. The pinwheel shape of both the dark boundary and the white cirrus suggests that the storm system rotates counterclockwise. Periodic small scale patterns in the white cloud, possibly waves, are short lived and do not persist from one Neptunian rotation to the next. (Courtesy NASA/JPL)
Until the Voyager 2 encounter in 1989, the rings surrounding Neptune were thought to be arcs. We now know that the rings completely circle the planet, but the thickness of each ring varies along its length. Neptune has a set of four rings which are narrow and very faint. The rings are made up of dust particles thought to have been made by tiny meteorites smashing into Neptune’s moons. From ground based telescopes the rings appear to be arcs but from Voyager 2 the arcs turned out to be bright spots or clumps in the ring system. The exact cause of the bright clumps is unknown.
The magnetic field of Neptune, like that of Uranus, is highly tilted at 47 degrees from the rotation axis and offset at least 13,500 kilometers or 8,500 miles from the physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of that planet’s sideways orientation or of any possible field reversals within the planet.
Neptune also has eight known satellites. Only two of these, Triton and Nereid, had been observed prior to the Voyager 2 flyby. Triton is the largest of the eight satellites and is almost as big as the Earth’s Moon. The other Neptunian satellites range in diameter from 58 to 416 km (36 to 258 miles). Apart from Triton, the moons of Neptune are irregularly shaped and have very dark surfaces.
Triton is the largest moon of Neptune, with a diameter of 2,700 kilometers (1,680 miles). It was discovered by William Lassell, a British astronomer, in 1846 scarcely a month after Neptune was discovered. Triton is colder than any other measured object in the Solar System with a surface temperature of -235° C (-391° F). It has an extremely thin atmosphere. Nitrogen ice particles might form thin clouds a few kilometers above the surface. The atmospheric pressure at Triton’s surface is about 14 microbars, 1/70,000th the surface pressure on Earth.
Triton is the only large satellite in the solar system to circle a planet in a retrograde direction — in a direction opposite to the rotation of the planet. It also has a density of about 2.066 grams per cubic centimeter (the density of water is 1.0 gram per cubic centimeter). This means Triton contains more rock in its interior than the icy satellites of Saturn and Uranus do. The relatively high density and the retrograde orbit has led some scientists to suggest that Triton may have been captured by Neptune as it traveled through space several billion years ago. If that is the case, tidal heating could have melted Triton in its originally eccentric orbit, and the satellite might even have been liquid for as long as one billion years after its capture by Neptune.
Triton is scarred by enormous cracks. Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometers into the atmosphere.
