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Planet X

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Planet X

  • /ˈplænɪt ɛks/

Where it comes from

X represents an unknown as opposed to the Roman numeral for 10 (initially, Planet X would have been the ninth planet).

proper noun

  1. A hypothetical planet in the Solar system, beyond the currently known outermost planet. (The term was applied to Eris for a short while, when Pluto was still considered the ninth planet.)
  2. obsolete A theoretical planet responsible for reported gravitational perturbations of Neptune's orbit. (This was initially believed to be Pluto. The reported perturbations were an error.)
  3. obsolete A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)

Translations

Every language

name hypothetical planet

Españolplaneta X
日本語惑星Ｘ wakusei X
中文X行星 X xíngxīng
PortuguêsPlaneta X
Françaisplanète X
DeutschTranspluto, Planet X
Italianopianeta X
РусскийПлане́та Х Planéta X, Плане́та Икс Planéta Iks
Tiếng Việthành tinh X

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Cite this
MLA

"Planet X." Wiktionary, via Micwyn, micwyn.com/word/planet%20x. Accessed 6 Nov. 2026.

APA

Planet X. (n.d.). In Wiktionary, via Micwyn. Retrieved November 6, 2026, from https://micwyn.com/word/planet%20x

Chicago

"Planet X." Wiktionary, via Micwyn. Accessed November 6, 2026. https://micwyn.com/word/planet%20x.

BibTeX
@misc{planet_x_2026,
  title = {Planet X},
  howpublished = {Wiktionary, via Micwyn},
  year = {2026},
  url = {https://micwyn.com/word/planet%20x},
  urldate = {2026-11-06}
}