planet X

American

[plan-it eks] / ˈplæn ɪt ˈɛks /

noun

Astronomy.

a planet beyond the orbit of Pluto whose existence has been hypothesized but not confirmed.

Etymology

Origin of planet X

First recorded in 1880–85

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

See Examples For:

planet X

Labelled as planet X, prominent astronomers had put it forward as an explanation for Uranus’s orbit, which drifts from the path of orbital motion that physics would expect it to follow.

From Science Daily ● Jun. 9, 2026

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023