Planet X is a large hypothetical planet beyond that of Neptune. The scientific basis of the Planet X hypothesis was broadly discounted in the early 1990s and today no significant portion of the scientific community believes it to exist.

The "X" mentioned in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for ten. At the time of its conception there were eight known planets in the solar system; its existence, first as a ninth planet, and then from 1930 until its demise as a tenth, was postulated on the basis of apparent discrepancies in the orbits of the gas giants, especially those of Uranus and Neptune. Those discrepancies have largely been resolved by modern measurement, removing the basis for Planet X.

Although Pluto was discovered as a result of the search for Planet X, it is not considered Planet X. Neither is Eris, even though it was at one point considered for reclassification as a planet under a proposal outlined by the International Astronomical Union (see 2006 redefinition of planet).

In popular culture, "Planet X" has become a generic stand-in term for an undiscovered planet in the solar system. Its use by scientists, however, is exclusively in reference to the particular hypothesis discussed below.

Origin of the discrepancy

At the beginning of the 20th century, many astronomers speculated about the existence of a planet beyond Neptune. The discovery of Neptune resulted from calculations of the mathematicians John Couch Adams and Urbain Le Verrier to explain discrepancies between the calculated and observed orbits of Uranus, Saturn, and Jupiter.

After the discovery of Neptune, however there still were some slight discrepancies in those orbits, and also in the orbit of Neptune itself. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.

Percival Lowell, who is most well known for his claims of having observed canals on Mars, called this hypothetical planet "Planet X". He performed two searches for it without success, the first ending in 1909, and after revising his prediction for where it should be found, the second from 1913 to 1915, after which Lowell published his mathematical hypothesis of the parameters for Planet X. Ironically, at his observatory that year, two faint images of Pluto were recorded, but were not recognized as planets at the time.

Discovery of Pluto

Lowell died in 1916, but in 1928 the Lowell Observatory began another search, which ended with the discovery of Pluto by Clyde Tombaugh in 1930.

Upon its discovery, Pluto was originally thought to be Planet X, but its mass was not sufficient to explain Neptune's orbit, so the search continued.

Further searches for Planet(s) X

After discovering Pluto, Tombaugh continued to search the ecliptic for other distant planets. He found asteroids, variable stars, and even a comet, but no more planets.

In the 1980s and 1990s, astronomer Robert Sutton Harrington of the US Naval Observatory, who had first calculated that Pluto was too small to have perturbed the orbits of Uranus and Neptune, led a search to determine the real cause of the planets' apparently irregular orbits. He calculated that any Planet X would be at roughly three times the distance from the sun of Neptune's orbit, highly elliptical, and far below the ecliptic (the planet's orbit would be at roughly a 90-degree angle from the orbit plane of the other known planets). This hypothesis was met with a mixed reception. Noted Planet X skeptic Brian Marsden of Harvard University's Minor Planet Center has pointed out that these discrepancies are a hundred times smaller than those noticed by Adams and Le Verrier, and could easily be due to observational error. Harrington died in 1993, having never found Planet X.

After Pluto and Charon (discovered in 1978), no more trans-Neptunian objects were found until the discovery of (15760) 1992 QB₁ in 1992. Since that time, hundreds of trans-Neptunian objects have been discovered. The objects are now recognized as mostly belonging to the Kuiper Belt: icy bodies orbiting in the plane of the ecliptic beyond Neptune, which are left over from the formation of the solar system. Pluto itself is now recognized as being a member of the Kuiper Belt, and the second largest dwarf planet. Pluto lost its status as a planet because it failed to meet the IAU definition of a planet, which would require it to have cleared the neighbourhood around its orbit.

Planet X disproved

The space age was to disprove the Planet X hypothesis, as understood by Lowell. After the Voyager 2 flyby of Neptune in 1989, a very accurate value for that planet's mass was obtained. When the newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in its orbit vanished.

Also, to date there are no discrepancies in the trajectories of any space probes (Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2) that can be attributed to the gravitational pull of a large undiscovered object in the outer solar system.

While many astronomers consider this the end of Lowell's Planet X hypothesis, it does not preclude the existence of objects potentially as massive as the Earth that could escape detection in this manner, and which would not cause detectable discrepancies in the orbits of the outer planets.

Planet X revived

The search for Planet X may not be over yet. The Kuiper Belt terminates suddenly at a distance of 55 AU from the Sun, and there is some speculation this may be caused by the presence of an object with a mass between that of Mars and Earth located beyond 55 AU.

An alternative theory, proposed by John Murray of the Open University and John Matese, Patrick Whitman and Daniel Whitmire of the University of Louisiana at Lafayette, has long period comets originating from specific regions of the sky, rather than coming from random directions as proposed by Oort. This would result from comets being disturbed by an unseen object at least as large as Jupiter, and possibly a brown dwarf.^[1]

References

1. ^ SETH BORENSTEIN (1999). A 10th planet may be out there or new object could be the sun's long-lost twin, astronomers say. Huston Chronicle. Retrieved on 2007-04-01.

• Ken Croswell: Planet Quest: The Epic Discovery of Alien Solar Systems, London, The Free Press, 1997 pp. 56-71
• P. K. Seidelmann, R. S. Harrington (1987). "Planet X - The current status". Journa Celestial Mechanics and Dynamical Astronomy 43 (1-4): 55-68. DOI:10.1007/BF01234554. 
• D. P. Whitmire, J. J. Matese (1985). "Periodic comet showers and planet X". Nature 313: 36 - 38. DOI:10.1038/313036a0. 
• G. D. Quinlan (1985). "Planet X: a myth exposed". Nature 363: 18 - 19. DOI:10.1038/363018b0. 
• xfacts.com

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