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planet X

American  
[plan-it eks] / ˈplæn ɪt ˈɛks /

noun

Astronomy.

  1. 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

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

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.