Planet X

::This article is about the hypothetical planet.

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::Planet X can also mean Planet 10. See tenth planet

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::For other meanings see Planet X (disambiguation)

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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.

Related Topics:
Hypothetical planet - Neptune - Orbit - Gas giant - Uranus

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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.

Related Topics:
Pluto - 2003 UB₃₁₃ - International Astronomical Union - Tenth planet

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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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