Meaning of Planet X | Babel Free

Definitions

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

Equivalents

CEFR level