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Planets beyond Neptune
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Percival Lowell, originator of the Planet X hypothesis
Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century and culminated at the start of the 20th with Percival Lowell's quest for Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the giant planets, particularly Uranus and Neptune, speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially named the ninth planet. In 1978, Pluto was conclusively determined to be too small for its gravity to affect the giant planets, resulting in a brief search for a tenth planet. The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass. After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet, or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Union reclassified Pluto and its largest neighbours as dwarf planets, leaving Neptune the farthest known planet in the Solar System.While today the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, the concept of an as yet unobserved planet has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. As of March 2014, observations with the WISE telescope have ruled out the possibility of a Saturn-sized object (95 Earth mass) out to 10,000 AU, and a Jupiter-sized or larger object out to 26,000 AU.In 2014, based on similarities of the orbits of a group of recently discovered extreme trans-Neptunian objects, astronomers hypothesized the existence of a super-Earth planet, 2 to 15 times the mass of the Earth and beyond 200 AU with possibly a high inclined orbit at some 1500 AU. In 2016 further work showed this unknown distant planet is likely on an inclined, eccentric orbit that goes no closer than about 200 AU and no further than about 1600 AU from the Sun. The orbit is predicted to be anti-aligned to the clustered extreme trans-Neptunian objects. Because Pluto is no longer considered a planet by the International Astronomical Union, this new hypothetical object has become known as Planet Nine.
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^ Tombaugh (1946), p. 73.
^ Tom Standage (2000). The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting. New York: Walker. p. 188. ISBN 978-0-8027-1363-6.
^ "IAU 2006 General Assembly: Resolutions 5 and 6" (PDF). International Astronomical Union. 2006-08-24.
^ S. C. Tegler & W. Romanishin (2001). "Almost Planet X". Nature 411 (6836): 423–424. doi:10.1038/35078164. PMID 11373654.
^ Luhman, K. L. (2014). "A Search for a Distant Companion to the Sun with the Wide-field Infrared Survey Explorer". The Astrophysical Journal 781 (1): 4. Bibcode:2014ApJ...781....4L. doi:10.1088/0004-637X/781/1/4.
^ Trujillo, C. A.; Sheppard, S. S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units" (PDF). Nature 507 (7493): 471–474. Bibcode:2014Natur.507..471T. doi:10.1038/nature13156. Archived (PDF) from the original on 2014-12-16.
^ http://iopscience.iop.org/article/10.3847/0004-6256/151/2/22 iop>
^ Burdick, Alan (20 January 2016). "Discovering Planet Nine". The New Yorker. Retrieved 20 January 2016.
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