One of the mysteries of the English language finally explained.
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. Also (especially Science Fiction): (an arbitrary designation for) an unknown or hypothetical alien planet.
Late 19th century. From planet + X.
Planet X/ˌplanɪt ˈɛks/
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