Planet X

/ˈplænɪt ɛks/

proper noun

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.)

Ask AI about this wordChatGPTClaudePerplexityGoogle AIGrok

Meanings

proper noun
  1. astronomyA 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. obsoleteastronomyA 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. obsoleteastronomyA theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)

Synonyms & antonyms

Also see
pluto

Origin

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

Full word history & related roots →

Translations

Chinese Cantonese X行星
  • X行星 (iks¹ hang⁴ sing¹, iks¹ haang⁴ sing¹)hypothetical planet
Chinese Mandarin X行星
  • X行星 (X xíngxīng)hypothetical planet
Dutch planeet X
  • planeet Xhypothetical planet
Finnish planeetta X
  • planeetta Xhypothetical planet
French planète X
  • planète Xhypothetical planet
German Transpluto, Planet X
  • Transplutohypothetical planet
  • Planet Xhypothetical planet
Italian pianeta X
  • pianeta Xhypothetical planet
Japanese 惑星Ｘ
  • 惑星Ｘ (wakusei X)hypothetical planet
Portuguese Planeta X
  • Planeta Xhypothetical planet
Russian Плане́та Х, Плане́та Икс
  • Плане́та Х (Planéta X)hypothetical planet
  • Плане́та Икс (Planéta Iks)hypothetical planet
Spanish planeta X
  • planeta Xhypothetical planet
Swedish planeten X
  • planeten Xhypothetical planet
Vietnamese hành tinh X
  • hành tinh Xhypothetical planet
Cite this entry
APA

Planet X. (n.d.). In WatkinsX Dictionary. Retrieved November 4, 2026, from https://dictionary.watkinsx.com/word/Planet%20X

MLA

“Planet X.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/Planet%20X. Accessed November 4, 2026.

Chicago

WatkinsX Dictionary, s.v. “Planet X,” accessed November 4, 2026, https://dictionary.watkinsx.com/word/Planet%20X.

Source: Planet X on Wiktionary, CC BY-SA 4.0.