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Planet X is a proposed planet beyond the remit of the planet Neptune.
Planet X is a large Hypothetical planet with an orbit beyond that of Neptune. The X mentioned in the name represents an "unknown", as opposed to the Roman numeral for ten. At the time of its conception there were only eight known planets in the Solar System. Planet X existence, first as a ninth planet, then, after 1930, as a tenth, and once again a ninth planet after Pluto was reclassified in 2006, 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. Nor is Scattered disc object object 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 of Planet X.
At the end of the 19th 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. However, after the Voyager 2 flyby of Neptune 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 suggestive discrepancies vanished.
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 a planet 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 G. 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 roughly three times Neptune's orbit, highly elliptical, and far below the ecliptic. 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, since it has not cleared the neighbourhood around its orbit.
Planet X disproved.
The distant space probes Pioneer 10, Pioneer 11, and Voyagers 1 and 2 disproved the existence of Planet X, as hypothesized by Lowell, in two ways. First, as they passed each of the outer planets, the acceleration gained by the gravitational pull of the planet was used to refine the masses of those planets. It turned out that the masses of the outer planets, calculated by Earth-based observatories, were low by as much as 1%. When the correct masses were used to determine the orbits of the outer planets, the remaining discrepancies vanished.
Second, the trajectories of the space probes have revealed no discrepancies that can be accounted for by the gravitational pull of a large undiscovered object in the solar system. Many astronomers consider this the end of the Planet X hypothesis. This does not preclude the existence of objects potentially as massive as the Earth, which could escape detection in this manner, and would not create noticeable discrepancies in the orbits of the outer planets. Such an object could be considered "Planet X" only in the popular sense, not in the scientific sense.
Planet X revived.
The story of the search for a Planet X may not be over yet. The Kuiper Belt comes to a sudden end at 55 AU, and there is speculation that this is caused by the presence of an object much larger than other objects in the Kuiper Belt, with a mass intermediate between those of Mars and Earth beyond 55 AU. (Strictly speaking, this is not the same Planet X, because it comes from a different hypothetical basis.)
Objects such as 50000 Quaoar, 90377 Sedna and Eris, discovered in 2002, 2004, and 2005, respectively, by California Institute of Technology scientists, are too small to fit this new Planet X hypothesis. Sedna is also too distant.
An alternative theory has been proposed by Doctor John Murray of the open university and by John Matese of the University of Louisiana at Lafayette. Both scientists claim to have observed that long period comets, rather than appearing at random from every corner of the sky as Jan Oort supposed, appear to be biased toward a certain region, which they believe could be due to their being disturbed by an unseen giant object. This object would be at least the size of Jupiter and probably more akin to a brown dwarf.
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