Planet X

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This article is about the hypothetical planet. For the comic book storyline, see Planet X (comics).
Planet X can also mean Planet 10. See tenth planet
For progressive rock band see Planet X (band)

Planet X is a large hypothetical planet orbiting beyond the orbit of Neptune. (The X stands for unknown, rather than the Roman number 10.) Its existence, first as a ninth planet and after 1930 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 Planet X. The newly discovered Kuiper Belt object 2003 UB313 is not Planet X either, although the International Astronomical Union may in the future designate it as a tenth 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 in this article.

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Origin of the discrepancy

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 Adams and 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. 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" (X for "unknown", not the Roman numeral "ten", as only eight planets were known at the time). 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.

After Pluto and Charon (discovered in 1978), no more trans-Neptunian objects were found until the discovery of Template:Mpl 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 one of the largest of the Kuiper Belt objects. This has caused some concern over whether Pluto should be called a planet, but at this time Pluto officially remains a planet.

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 would only be Planet X in the popular sense, not a scientific one.

Beyond the Kuiper Belt

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 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 Template:Mpl, 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.

Planet X in fiction

  • In the sixth Godzilla film, Monster Zero, aliens known in Japan as X-seijin and in America as Xians hail from Planet X, between Jupiter and Saturn.
  • In some Looney Tunes cartoons set in space, such as Duck Dodgers in the 24 1/2 century, there is a "Planet X". The name is noted by a bland sign sticking out saying PLANET X, a giant letter X on the planet's surface, or the planet having an X shape. In a few instances, Planet X is accompanied by planets named after other letters, such as Planet Q or Planet Z.
  • The computer RPG Ultima II has a Planet X that must be visited to acquire an artifact.

See also

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References

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