Measurements by Voyager 2 in extreme-ultraviolet and radio frequencies revealed that Neptune has faint and weak but complex and unique aurorae; however, these observations were limited in time and did not contain infrared. Subsequent astronomers using the Hubble Space Telescope have not glimpsed the aurorae, in contrast to the more well-defined aurorae of Uranus. [96] [97] In 1845–1846, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no enthusiasm in his compatriots. In June1846, upon seeing LeVerrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded James Challis to search for the planet. Challis vainly scoured the sky throughout August and September. [35] [38] Challis had, in fact, observed Neptune a year before the planet's subsequent discoverer, Johann Gottfried Galle, and on two occasions, 4 and 12 August 1845. However, his out-of-date star maps and poor observing techniques meant that he failed to recognise the observations as such until he carried out later analysis. Challis was full of remorse but blamed his neglect on his maps and the fact that he was distracted by his concurrent work on comet observations. [39] [35] [40] The formation of the ice giants, Neptune and Uranus, has proven difficult to model precisely. Current models suggest that the matter density in the outer regions of the Solar System was too low to account for the formation of such large bodies from the traditionally accepted method of core accretion, and various hypotheses have been advanced to explain their formation. One is that the ice giants were not formed by core accretion but from instabilities within the original protoplanetary disc and later had their atmospheres blasted away by radiation from a nearby massive OB star. [70]

Main article: Rings of Neptune Neptune's rings and moons viewed in infrared by the James Webb Space Telescope Neptune resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55radius, or about 13,500km from the planet's physical centre. Before Voyager 2 's arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective fluid motions in a thin spherical shell of electrically conducting liquids (probably a combination of ammonia, methane and water) [85] resulting in a dynamo action. [92] Neptune's spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and ethyne. [21] [28] The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide. [21] [89] The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons. [21] Neptune's atmosphere is subdivided 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, lies at a pressure of 0.1 bars (10kPa). [21] The stratosphere then gives way to the thermosphere at a pressure lower than 10 −5 to 10 −4 bars (1 to 10Pa). [21] The thermosphere gradually transitions to the exosphere. Neptune has such a long journey around the Sun it takes 165 Earth years to go around once. That’s a long year!

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Internal heating Four images taken a few hours apart with the NASA/ESA Hubble Space Telescope 's Wide Field Camera 3. Near infrared radiation data has been used as red channel. [114] From its discovery in 1846 until the discovery of Pluto in 1930, Neptune was the farthest known planet. When Pluto was discovered, it was considered a planet, and Neptune thus became the second-farthest known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer than Neptune to the Sun, making Neptune the ninth planet from the Sun during this period. [62] [63] The increasingly accurate estimations of Pluto's mass from ten times that of Earth's to far less than that of the Moon [64] and the discovery of the Kuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt. [65] [66] 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 outermost-known planet in the Solar System. [67] Physical characteristics A size comparison of Neptune and Earth

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980. The long orbital period of Neptune results in seasons lasting forty years. [104] Storms A second symbol, an ‘LV’ monogram for 'LeVerrier', analogous to the ‘H’ monogram for Uranus. It was never much used outside of France and is now archaic. Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt but far larger, extending from Neptune's orbit at 30AU out to about 55AU from the Sun. [125] Much in the same way that Jupiter's gravity dominates the asteroid belt, shaping its structure, so Neptune's gravity dominates the Kuiper belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune's gravity, creating gaps in its structure. The region between 40 and 42AU is an example. [126]Orbital elements refer to the Neptune barycentre and Solar System barycentre. These are the instantaneous osculating values at the precise J2000 epoch. Barycentre quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from the motion of the moons. Since 2018, the China National Space Administration has been studying a concept for a pair of Voyager-like interstellar probes tentatively known as Interstellar Express or Interstellar Heliosphere Probe. [172] Both probes will be launched at the same time in 2024 and take differing paths to explore opposing ends of the heliosphere; the second probe, IHP-2, will fly by Neptune in January 2038, passing only 1,000km above the cloud tops, and potentially carry an atmospheric impactor to be released during its approach. [173] Afterward, it will continue on its mission throughout the Kuiper belt toward the tail of the heliosphere, so far unexplored. The average distance between Neptune and the Sun is 4.5 billion km (about 30.1 astronomical units (AU)), and it completes an orbit on average every 164.79years, subject to a variability of around ±0.1years. The perihelion distance is 29.81AU; the aphelion distance is 30.33AU. [g] The first of these planetary rings was detected in 1968 by a team led by Edward Guinan. [30] [152] In the early 1980s, analysis of this data along with newer observations led to the hypothesis that this ring might be incomplete. [153] Main articles: Formation and evolution of the Solar System and Nice model A simulation showing the outer planets and Kuiper belt: a) before Jupiter and Saturn reached a 2:1 resonance; b) after inward scattering of Kuiper belt objects following the orbital shift of Neptune; c) after ejection of scattered Kuiper belt bodies by Jupiter

Neptune's 164-year orbital period means that the planet takes an average of 13 years to move through each constellation of the zodiac. In 2011, it completed its first full orbit of the Sun since being discovered and returned to where it was first spotted northeast of Iota Aquarii. [41] Neptune's mass of 1.0243 ×10 26kg [7] is intermediate between Earth and the larger gas giants: it is 17 times that of Earth but just 1/19th that of Jupiter. [f] Its gravity at 1 bar is 11.15m/s 2, 1.14 times the surface gravity of Earth, [68] and surpassed only by Jupiter. [69] Neptune's equatorial radius of 24,764km [10] is nearly four times that of Earth. Neptune, like Uranus, is an ice giant, a subclass of giant planet, because they are smaller and have higher concentrations of volatiles than Jupiter and Saturn. [70] In the search for exoplanets, Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes", [71] just as scientists refer to various extrasolar bodies as "Jupiters".The abundance of methane, ethane and acetylene at Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles because photochemistry cannot account for the distribution without meridional circulation. [21] Neptune brightened about 10% between 1980 and 2000 mostly due to the changing of the seasons. [159] Neptune may continue to brighten as it approaches perihelion in 2042. The apparent magnitude currently ranges from 7.67 to 7.89 with a mean of 7.78 and a standard deviation of 0.06. [15] Prior to 1980, the planet was as faint as magnitude 8.0. [15] Neptune is too faint to be visible to the naked eye. It can be outshone by Jupiter's Galilean moons, the dwarf planet Ceres and the asteroids 4 Vesta, 2 Pallas, 7 Iris, 3 Juno, and 6 Hebe. [160] A telescope or strong binoculars will resolve Neptune as a small blue disk, similar in appearance to Uranus. [161] Neptune differs from Uranus in its typical level of meteorological activity. Voyager 2 observed weather phenomena on Neptune during its 1989 flyby, [101] but no comparable phenomena on Uranus during its 1986 fly-by. 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. [154] Images from Voyager 2 in 1989 settled the issue by showing several faint rings.