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Definition of Planet X
1. Proper noun. (astronomy) A hypothetical planet in our solar system. (gloss Generally considered the next planet beyond the currently known outermost planet.) ¹
2. Proper noun. (astronomy obsolete) A theoretical planet that gravitationally perturbs the orbit of Neptune. (gloss This was incorrectly applied to Pluto. Later it became another planet further out to account for the missing mass that Pluto did not provide.) ¹
3. Proper noun. (astronomy obsolete) A theoretical planet that gravitationally perturbs the orbit of Uranus. (gloss This became Neptune.) ¹
4. Proper noun. (astronomy obsolete) A hypothetical tenth planet, beyond Pluto (qualifier 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.) ¹
¹ Source: wiktionary.com
Lexicographical Neighbors of Planet X
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