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NEPTUNE
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. Among the gaseous planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune.[c] Neptune orbits the Sun at an average distance of 30.1 astronomical units. Named after the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune was the first and only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject togravitational 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 13 moons were located telescopically until the 20th century. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.[10]
Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, like Jupiter's and Saturn's, is composed primarily ofhydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of "ices" such as water, ammonia, and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" to emphasise this distinction.[11] The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.[12] Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing.[13] Traces of methane in the outermost regions in part account for the planet's blue appearance.[14]
In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on 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).[15] 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 55 K(−218 °C). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C).[16][17] Neptune has a faint and fragmented ring system (labelled "arcs"), which may have been detected during the 1960s but was indisputably confirmed only in 1989 by Voyager 2.
Some of the earliest recorded observations ever made through a telescope, Galileo's drawings on 28 December 1612 and 27 January 1613, contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky;[19] hence, he is not credited with Neptune's discovery. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day. This apparent backward motion is created when Earth's orbit takes it past an outer planet. Because 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.[20] 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.[21]
In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus.[22] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction.[23] In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via Cambridge Observatory director 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.[24][25]
In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no 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 Challis to search for the planet. Challis vainly scoured the sky throughout August and September.[23][26]
Meanwhile, 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 aplanet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, 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.[23][27]
In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who 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.[28] 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.
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. Among the gaseous planets in the Solar System, Neptune is the most dense. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth, and not as dense as Neptune.[c] Neptune orbits the Sun at an average distance of 30.1 astronomical units. Named after the Roman god of the sea, its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune was the first and only planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject togravitational 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 13 moons were located telescopically until the 20th century. Neptune was visited by Voyager 2, when it flew by the planet on 25 August 1989.[10]
Neptune is similar in composition to Uranus, and both have compositions that differ from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, like Jupiter's and Saturn's, is composed primarily ofhydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of "ices" such as water, ammonia, and methane. Astronomers sometimes categorise Uranus and Neptune as "ice giants" to emphasise this distinction.[11] The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.[12] Perhaps the core has a solid surface, but the temperature would be thousands of degrees and the atmospheric pressure crushing.[13] Traces of methane in the outermost regions in part account for the planet's blue appearance.[14]
In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on 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).[15] 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 55 K(−218 °C). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C).[16][17] Neptune has a faint and fragmented ring system (labelled "arcs"), which may have been detected during the 1960s but was indisputably confirmed only in 1989 by Voyager 2.
Some of the earliest recorded observations ever made through a telescope, Galileo's drawings on 28 December 1612 and 27 January 1613, contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a fixed star when it appeared close—in conjunction—to Jupiter in the night sky;[19] hence, he is not credited with Neptune's discovery. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned retrograde that day. This apparent backward motion is created when Earth's orbit takes it past an outer planet. Because 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.[20] 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.[21]
In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus.[22] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational interaction.[23] In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via Cambridge Observatory director 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.[24][25]
In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but aroused no 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 Challis to search for the planet. Challis vainly scoured the sky throughout August and September.[23][26]
Meanwhile, 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 aplanet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, 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.[23][27]
In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who 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.[28] 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.