Planet X
/ˌplanɪt ˈɛks/Astronomy
noun
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.
Origin
Late 19th century. From planet + X.