Planet X

English

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

(astronomy) 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.)
(astronomy, 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.)
(astronomy, obsolete) A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)

hypothetical planet

Chinese:
Cantonese: X行星 (iks¹ hang⁴ sing¹, iks¹ haang⁴ sing¹)

Mandarin: X行星 (X xíngxīng)
Dutch: planeet X f
Finnish: planeetta X
French: planète X f
German: Transpluto m, Planet X m
Italian: pianeta X m
Japanese: 惑星Ｘ (wakusei X)
Maltese: pjaneta X
Portuguese: Planeta X m
Russian: Плане́та Х f (Planéta X), Плане́та Икс f (Planéta Iks)
Slovene: planet X m
Spanish: planeta X m
Swedish: planeten X c
Vietnamese: hành tinh X

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