/ Dictionary / Index P planet X: Meaning and Definition of Find definitions for: planet X Pronunciation: (eks), [key] — Astron. Astron. a tenth planet beyond the orbit of Pluto whose existence has been hypothesized but not confirmed. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. planet-struck planform Related Content Daily Word Quiz: cygnet Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language