Planet X
/ˈplænɪt ɛks/
Planet X proper noun: 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.).
Proper noun
Planet X
- 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.)
- 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.)
- A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)
Planet X meaning in Vietnamese
Planet X in Vietnamese
- hypothetical planet
- hành tinh X
Origin of Planet X
X represents an unknown as opposed to the Roman numeral for 10 (initially, Planet X would have been the ninth planet).
Questions about Planet X
- What does Planet X mean?
- Planet X means 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.).
- What is Planet X in Vietnamese?
- Planet X in Vietnamese is hành tinh X. See Planet X meaning in Vietnamese.
- What part of speech is Planet X?
- Planet X is a proper noun.
- How do you pronounce Planet X?
- Planet X is pronounced /ˈplænɪt ɛks/.
Words near Planet X in the dictionary
Source and license
Adapted from Wiktionary: Planet X (page history and authors). Text available under the Creative Commons Attribution-ShareAlike 4.0 license. Word.ws reformatted the entry and added links. Wiktionary data of 2026-09-02.
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Cite this entry: “Planet X.” Word.ws, https://word.ws/planet_x. Based on Wiktionary, CC BY-SA 4.0.