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Planets beyond Neptune
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== Early speculation == {{see also|Discovery of Neptune}} [[File:Jacques Babinet.jpg|thumb|right|[[Jacques Babinet]], an early proponent of a trans-Neptunian planet]] In the 1840s, the French mathematician [[Urbain Le Verrier]] used [[Classical mechanics|Newtonian mechanics]] to analyse perturbations in the orbit of Uranus, and hypothesised that they were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer [[Johann Gottfried Galle]]. On 23 September 1846, the night following his receipt of the letter, Galle and his student [[Heinrich Louis d'Arrest|Heinrich d'Arrest]] discovered Neptune, exactly where Le Verrier had predicted.<ref>Croswell (1997), p. 43</ref> There remained some slight discrepancies in the [[giant planet]]s' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune. Even before Neptune's discovery, some speculated that one planet alone was not enough to explain the discrepancy. On 17 November 1834, the British amateur astronomer the Reverend [[Thomas John Hussey]] reported a conversation he had had with French astronomer [[Alexis Bouvard]] to [[George Biddell Airy]], the British Astronomer Royal. Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with [[Peter Andreas Hansen]], director of the [[Gotha Observatory|Seeberg Observatory]] in [[Gotha]], about the subject. Hansen's opinion was that a single body could not adequately explain the motion of Uranus, and postulated that two planets lay beyond Uranus.<ref name="grosser">{{Cite journal |last=Grosser |first=Morton |date=June 1964 |title=The Search for a Planet beyond Neptune |journal=Isis |language=en |volume=55 |issue=2 |pages=163–183 |doi=10.1086/349825 |issn=0021-1753 |jstor=228182 |s2cid=144255699}}</ref> In 1848, [[Jacques Babinet]] raised an objection to Le Verrier's calculations, claiming that Neptune's observed mass was smaller and its orbit larger than Le Verrier had initially predicted. He postulated, based largely on simple subtraction from Le Verrier's calculations, that another planet of roughly 12 Earth masses, which he named "Hyperion", must exist beyond Neptune.<ref name=grosser/> Le Verrier denounced Babinet's hypothesis, saying, "[There is] absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part."<ref name=grosser/> In 1850 [[James Ferguson (American astronomer)|James Ferguson]], Assistant Astronomer at the [[United States Naval Observatory]], noted that he had "lost" a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet. Subsequent searches failed to recover the "planet" in a different position, and in 1878, [[CHF Peters]], director of the [[Hamilton College]] Observatory in [[New York (state)|New York]], showed that the star had not in fact vanished, and that the previous results had been due to human error.<ref name=grosser/> In 1879, [[Camille Flammarion]] noted that the comets [[Comet Swift–Tuttle|1862 III]] and [[177P/Barnard|1889 III]] had [[Apsis|aphelia]] of 47 and 49 [[Astronomical unit|AU]], respectively, suggesting that they might mark the orbital radius of an unknown planet that had dragged them into an elliptical orbit.<ref name=grosser/> Astronomer [[George Forbes (scientist)|George Forbes]] concluded on the basis of this evidence that two planets must exist beyond Neptune. He calculated, based on the fact that four comets possessed aphelia at around 100 AU and a further six with aphelia clustered at around 300 AU, the orbital elements of a pair of hypothetical trans-Neptunian planets. These elements accorded suggestively with those made independently by another astronomer named [[David Peck Todd]], suggesting to many that they might be valid.<ref name=grosser/> However, sceptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results.<ref name=grosser/> Some have considered Forbes's hypothesis a precursor to [[Planet Nine]].<ref>{{cite journal|title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean Motion Resonances|journal=The Astronomical Journal|volume=153|issue=3|pages=91|year=2017|last1=Millholland|first1=Sarah|last2=Laughlin|first2=Gregory|doi=10.3847/1538-3881/153/3/91|arxiv=1612.07774|bibcode=2017AJ....153...91M|s2cid=119325788 |doi-access=free }}</ref> In 1900 and 1901, [[Harvard College Observatory]] director [[William Henry Pickering]] led two searches for trans-Neptunian planets. The first was begun by Danish astronomer [[Hans E. Lau|Hans Emil Lau]] who, after studying the data on the orbit of Uranus from 1690 to 1895, concluded that one trans-Neptunian planet alone could not account for the discrepancies in its orbit, and postulated the positions of two planets he believed were responsible. The second was launched when Gabriel Dallet suggested that a single trans-Neptunian planet lying at 47 AU could account for the motion of Uranus. Pickering agreed to examine plates for any suspected planets. In neither case were any found.<ref name=grosser/> In 1902, after observing the orbits of comets with aphelia beyond Neptune, Theodor Grigull of [[Münster]], Germany proclaimed the existence of a Uranus-sized planet at 50 AU with a 360-year period, which he named Hades, cross-checking with the deviations in the orbit of Uranus. In 1921, Grigull revised his orbital period to 310–330 years, to better fit the observed deviations.<ref>{{Cite web |last=Schlyter |first=Paul |title=Hypothetical Planets |url=http://solarviews.com/eng/hypothet.htm |access-date=2019-12-07 |website=The Swedish Amateur Astronomical Society}}</ref> In 1909, [[Thomas Jefferson Jackson See]], an astronomer with a reputation as an egocentric contrarian, opined "there is certainly one, most likely two and possibly three planets beyond Neptune".<ref name="see">{{Cite journal |last=Sherrill |first=Thomas J. |date=February 1999 |title=A Career of Controversy: The Anomaly of T.J.J. See |journal=Journal for the History of Astronomy |language=en |volume=30 |issue=1 |pages=25–50 |bibcode=1999JHA....30...25S |doi=10.1177/002182869903000102 |issn=0021-8286 |s2cid=117727302}}</ref> Tentatively naming the first planet "Oceanus", he placed their respective distances at 42, 56 and 72 AU from the Sun. He gave no indication as to how he determined their existence, and no known searches were mounted to locate them.<ref name=see/> In 1911, Indian astronomer [[Venkatesh P. Ketakar]] suggested the existence of two trans-Neptunian planets, which he named after the Hindu gods [[Brahma]] and [[Vishnu]], by reworking the patterns observed by [[Pierre-Simon Laplace]] in the planetary satellites of [[Jupiter]] and applying them to the outer planets.<ref name=ketakar/> The three inner [[Galilean moons]] of Jupiter, [[Io (moon)|Io]], [[Europa (moon)|Europa]] and [[Ganymede (moon)|Ganymede]], are locked in a complicated 1:2:4 resonance called a [[Orbital resonance#The Laplace resonance|Laplace resonance]].<ref name="Musotto2002">{{Cite journal |last1=Musotto |first1=Susanna |last2=Varadi |first2=Ferenc |last3=Moore |first3=William |last4=Schubert |first4=Gerald |date=October 2002 |title=Numerical Simulations of the Orbits of the Galilean Satellites |journal=Icarus |language=en |volume=159 |issue=2 |pages=500–504 |bibcode=2002Icar..159..500M |doi=10.1006/icar.2002.6939}}</ref> Ketakar suggested that Uranus, Neptune and his hypothetical trans-Neptunian planets were also locked in Laplace-like resonances. This is incorrect; Uranus and Neptune, while in a near-2:1 resonance, are not in full resonance.<ref>{{Cite journal |last=Malhotra |first=Renu |author-link=Renu Malhotra |year=1998 |title=Orbital Resonances and Chaos in the Solar System |url=https://www.lpl.arizona.edu/~renu/malhotra_preprints/rio97.pdf |journal=Solar System Formation and Evolution ASP Conference Series |volume=149 |page=37 |bibcode=1998ASPC..149...37M}}</ref> His calculations predicted a mean distance for Brahma of 38.95 AU and an orbital period of 242.28 Earth years (3:4 resonance with Neptune). When Pluto was discovered 19 years later, its mean distance of 39.48 AU and orbital period of 248 Earth years were close to Ketakar's prediction (Pluto in fact has a [[Plutino|2:3 resonance with Neptune]]). Ketakar made no predictions for the orbital elements other than mean distance and period. It is not clear how Ketakar arrived at these figures, and his second planet, Vishnu, was never located.<ref name="ketakar">{{Cite journal |last1=Chhabra |first1=J. G. |last2=Sharma |first2=S. D. |last3=Khanna |first3=Manju |date=1984 |title=Prediction of Pluto by V. P. Ketakar |url=http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_1/20005abd_18.pdf |url-status=dead |journal=Indian Journal of History of Science |volume=19 |issue=1 |pages=18–26 |bibcode=1984InJHS..19...18C |archive-url=https://web.archive.org/web/20090225135119/http://www.new.dli.ernet.in/rawdataupload/upload/insa/INSA_1/20005abd_18.pdf |archive-date=2009-02-25 |access-date=2008-09-04}}</ref>
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