Planet X

Etymology
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.
 * 2)  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)  A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. This planet was discovered and is now known as Neptune.

Translations

 * Chinese:
 * Cantonese: X行星
 * Mandarin: X行星
 * Dutch: planeet X
 * Finnish: planeetta X
 * French: planète X
 * German: Transpluto, Planet X
 * Italian: pianeta X
 * Japanese: 惑星X
 * Maltese: pjaneta X
 * Portuguese: Planeta X
 * Russian: Плане́та Х, Плане́та Икс
 * Slovene: planet X
 * Spanish: planeta X
 * Swedish: planeten X
 * Vietnamese: hành tinh X