Diameter: 49,500 kilometers
Temperature: -200 °C (72 K)
Orbit: Takes 164.8 Earth-years to complete an orbit.
Average Distance:  4,500 million kilometers - 2,794.4 (millions of miles) from Sun
Mass:  17.15 Earth-masses
Moons: 13
Period of Rotation: 17.7 hours

Missions to Neptune:

Voyager 2's closest approach to Neptune occurred on August 25, 1989

Neptune in history:

Galileo was the first to observe Neptune on December 28, 1612 and again on January 27, 1613 but in both occasions, Galileo mistook Neptune for a fixed star. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned retrograde that very day. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet.  In July 2009 University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was at least aware that the star he had observed had moved relative to the fixed stars.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbor planet Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction. In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via James Challis, he requested from Sir George Airy, the Astronomer Royal, who sent the data in February 1844. Adams continued to work on this in 1845-1846 and produced several different estimates of new planet, but did not respond to a request from Airy about the orbit of Uranus.

In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but also experienced difficulties in stimulating any enthusiasm in his compatriots. In June 1846, upon seeing Le Verrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded Cambridge Observatory director James Challis to search for the planet. Challis vainly scoured the sky throughout August and September.

Meantime, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. The very evening of the day of receipt of Le Verrier's letter on September 23, 1846, Neptune was discovered within 1° of where Le Verrier had predicted it to be, and about 12° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.

In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. Since 1966 Dennis Rawlins has questioned the credibility of Adams's claim to co-discovery and the issue was re-evaluated by historians with the return in 1998 of the "Neptune papers" (historical documents) to the Royal Observatory, Greenwich. After reviewing the documents, they suggest that "Adams does not deserve equal credit with Le Verrier for the discovery of Neptune. That credit belongs only to the person who succeeded both in predicting the planet's place and in convincing astronomers to search for it."

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus.

Claiming the right to name his discovery, Le Verrier quickly proposed the name Neptune for this new planet, while falsely stating that this had been officially approved by the French Bureau des Longitudes. In October, he sought to name the planet Le Verrier, after himself, and he had loyal support in this from the observatory director, François Arago. This suggestion met with stiff resistance outside France. French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet.

Struve came out in favour of the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune became the internationally accepted name. In Roman mythology, Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Earth, were named for Greek and Roman mythology.

Most languages today, even in countries that have no direct link to Graeco-Roman culture, use some variant of the name "Neptune" for the planet; in Chinese, Japanese and Korean, the planet's name was literally translated as "sea king star" (海王星), since Neptune was the god of the sea.

Neptune internal structure: 

Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10 to 20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere.

The mantle reaches temperatures of 2,000 K to 5,000 K. It is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core. The mantle may consist of a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times that of the Earth. The pressure at the centre is 7 Mbar (700 GPa), millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.

Rings of Neptune:

Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The three main rings are the narrow Adams Ring, 63000 km from the centre of Neptune, the Le Verrier Ring, at 53000 km, and the broader, fainter Galle Ring, at 42000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000 km.

The first of these planetary rings was discovered in 1968 by a team led by Edward Guinan, but it was later thought that this ring might be incomplete. Evidence that the rings might have gaps first arose during a stellar occultation in 1984 when the rings obscured a star on immersion but not on emersion. Images by Voyager 2 in 1989 settled the issue by showing several faint rings. These rings have a clumpy structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.

The outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité (Courage, Liberty, Equality and Fraternity). The existence of arcs was difficult to explain because the laws of motion would predict that arcs would spread out into a uniform ring over very short timescales. Astronomers now believe that the arcs are corralled into their current form by the gravitational effects of Galatea, a moon just inward from the ring.

Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. Images taken from the W. M. Keck Observatory in 2002 and 2003 show considerable decay in the rings when compared to images by Voyager 2. In particular, it seems that the Liberté arc might disappear in as little as one century.

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