Planet X n.

(an arbitrary designation for) an unknown or hypothetical alien planet

In early use, in Astronomy: a hypothetical undiscovered planet in the solar system, whose supposed orbit lies beyond that of the outermost planet known; (originally) that identified as the ninth planet and named Pluto; (later) a hypothetical planet or planets beyond Pluto


  • [1882 A. R. Wilkie Rosa xvii. 130

    Planet X, that huge and glorious constellation sweeping in the circuit of infinite space, will be realized to be the future home of the children of men.]

  • [1930 Times 15 Mar. 12/7

    The distance of β€˜planet X’ from the sun has been estimated at Flagstaff as 45 times the distance of the earth from the sun.]

  • 1976 J. L. Christian Extra-Terrestrial Intelligence 202

    We can only speculate about knowledge on Planet X if we speculate on the general way its inhabitants live and relate.

  • 1991 J. Varley Steel Beach (1993) 259 John Varley bibliography

    The scrambler denatured her voice, made her sound like a creepoid from Planet X.

  • 2011 M. Atwood Burning Bushes: Why Heaven and Hell Went to Planet X in In Other Worlds: SF & the Human Imagination 45 page image Margaret Atwood bibliography

    [H]earing a bush speak...could happen easily in a fairy tale, however, or a β€˜fable’ like Alice in Wonderland, or in a Greek myth at the point at which some hapless maiden is being transformed into a sapling or other item of vegetation by a god. And it could happen effortlessly on Planet X.

Research requirements

antedating 1976

Research History
Ben Ostrowsky submitted a 2011 cite from Margaret Atwood.

Last modified 2021-02-06 21:13:56
In the compilation of some entries, HDSF has drawn extensively on corresponding entries in OED.