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Planet X (or Transpluto) is a hypothetical planet beyond Pluto. Its existence was argued for on the basis of apparent discrepancies in the orbit of Neptune. A common name for this hypothetical planet is Persephone.
Reasons for Planet X's existence
Many astronomers, at the end of the 19th century, speculated about the existence of a Planet X. The reason for this enthusiasm was that, less than 50 years before, the very planet Neptune was discovered following the direction of the mathematicians Adams and Le Verrier, who based their calculation on discrepancies on the orbits of Uranus, Saturn and Jupiter.
If a planet was so spectacularly found just calculating the differences from theoretical and real orbitals of already known planets, they reasoned, there was a real possibility that the errors in Neptune's orbit could be explained by a new, unknown planet.
The search for Planet X
Percival Lowell, who is most well known as a proponent for canals on Mars, called this hypothetical planet "Planet X" (X for unknown). He performed two searches for it, without success, the first ending in 1909, and the second started in 1913 after revising his prediction for where it should be found. This search ended in 1915, after which Lowell published his theoretical results parameters for Planet X. Ironically, at his observatory that very same year, two faint images of Pluto were recorded, but not recognized as a planet until after the discovery of Pluto in 1930.
Search conclusion
Pluto was originally thought to be Planet X, but Pluto's mass was not sufficient to explain Neptune's orbit, so the search continued. However these apparent discrepancies were resolved when the Voyager 2 space probe discovered that Neptune's mass had been miscalculated; with Neptune's newly discovered mass taken into account, there was no longer a need for any new planet to explain Neptune's orbit.
Possible other Planet Xs
Our most powerful detection techniques are capable of detecting an Earth-sized planet 70 AU from the Sun, a Uranus-sized planet 90 AU away, and a Jupiter-sized planet up to 120 AU away (neglecting its gravitational effects on the Sun). Of course, the sky is very big and the most powerful telescopes can only look at a very tiny fraction of it at a time. Pluto, for comparison, is around 30 AU away at the moment.
If a tenth planet exists, it is unlikely to be native to the solar system: comprehensive surveys of the ecliptic have been undertaken, concluding that no planet of Earth size or greater exists in the ecliptic plane closer than 60 AU. Thus, any tenth planet would have to be in a highly inclined orbit, and so likely to be a captured object and not one that was formed with the solar system. Several large objects (smaller than Pluto, but larger than 1 Ceres) such as 90377 Sedna and (90482) 2004 DW have been discovered, but these are not the target of the search, and nearly all astronomers consider them too small to be considered planets.
Planet X in Fiction
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