Planet X

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

• IPA^(key): /ˈplænɪt ɛks/

Planet X

1. (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.)
2. (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.)
3. (astronomy, obsolete) A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)

• Nibiru
• Tiamat

• explant