Planet X

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• proper noun astronomy A hypothetical planet in our solar system. (Generally considered the next planet beyond the currently known outermost planet.)
• proper noun astronomy, obsolete A theoretical planet that gravitationally perturbs the orbit of Neptune. (This was incorrectly applied to Pluto. Later it became another planet further out to account for the missing mass that Pluto did not provide.)
• proper noun astronomy, obsolete A theoretical planet that gravitationally perturbs the orbit of Uranus. (This became Neptune.)
• proper noun astronomy, obsolete A hypothetical tenth planet, beyond Pluto (Pluto was demoted to minor planet status as a dwarf planet, so there are only eight planets. It had been applied to Eris for a short while.)