Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is somewhat more massive than its near-twin Uranus, which is 15 times the mass of Earth but not as dense.[12] On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the Earth–Sun distance. Named for the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus ledAlexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed on 23 September 1846[1] by Johann Galle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 12 moons were located telescopically until the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on 25 August 1989.
Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces ofhydrocarbons and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" in order to emphasise these distinctions.[13] The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.[14] Traces of methane in the outermost regions in part account for the planet's blue appearance.[15]
In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphere is notable for its active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spoton Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (1,300 mph).[16] Because of its great distance from the Sun, Neptune's outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C (55 K). Temperatures at the planet's centre are approximately 5,400 K(5,000 °C).[17][18] Neptune has a faint and fragmented ring system (labeled 'arcs'), which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2.[19]
Discovery
Main article: Discovery of Neptune
Galileo's drawings show that he first observed Neptune on 28 December 1612, and again on 27 January 1613. On both occasions, Galileo mistook Neptune for a fixed star when it appeared very close—in conjunction—to Jupiter in the night sky;[20] hence, he is not credited with Neptune's discovery. 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. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small telescope.[21] 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.[22]
In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus.[23] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction.[24] In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via James Challis, he requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams continued to work in 1845–46 and produced several different estimates of a new planet.[25][26]
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 Observatorydirector James Challis to search for the planet. Challis vainly scoured the sky throughout August and September.[24][27]
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 23 September 1846, Neptune was discovered within 1° of where Le Verrier had predicted it to be, and about 12° from Adams' prediction. Challis later realised that he had observed the planet twice in August (Neptune had been observed on 8 and 12 August, but because Challis lacked an up-to-date star-map it was not recognised as a planet), failing to identify it owing to his casual approach to the work.[24][28]
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.[29] 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."[30]
Naming
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.[31]
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.[32] 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.[33] French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet.[34]
Struve came out in favour of the name Neptune on 29 December 1846, to the Saint Petersburg Academy of Sciences.[35] 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 andRoman mythology.[36]
Most languages today, even in countries that have no direct link to Greco-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.[37] In modern Greek, though, the planet retains its ancient name Poseidon (Ποσειδώνας: Poseidonas), the Greek counterpart to Neptune.[38]
Status
From its discovery in 1846 until the subsequent discovery of Pluto in 1930, Neptune was the farthest known planet. Upon Pluto's discovery Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer to the sun than Neptune.[39] The discovery of the Kuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet in its own right or part of the belt's larger structure.[40][41] In 2006, the International Astronomical Union defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet" and making Neptune once again the last planet in the Solar System.[42]
With a mass of 1.0243×1026 kg,[5] Neptune is an intermediate body between Earth and the larger gas giants: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter.[12] The planet's surface gravity is only surpassed by Jupiter.[43] Neptune's equatorial radius of 24764 km[9] is nearly four times that of the Earth. Neptune and Uranus are often considered a sub-class of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatilesrelative to Jupiter and Saturn.[44] In the search for extrasolar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes",[45] just as astronomers refer to various extra-solar bodies as "Jupiters".
Internal structure
Neptune's internal structure resembles that of Uranus. Its atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% 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.[17]
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.[1] 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.[46] At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.[47] 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.[48]
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.[49] The pressure at the centre is 7 Mbar (700 GPa), about twice as high as that at the centre of the Earth, and the temperature may be 5,400 K.[17][18]
At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.[17] A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,[50] although Neptune's vivid azure differs from Uranus's milder cyan. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.[15]
Neptune's atmosphere is sub-divided into two main regions; the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, occurs at a pressure of 0.1 bars (10 kPa).[13] The stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa).[13] The thermosphere gradually transitions to the exosphere.