Definition of Planet X in US English:

Planet X

noun

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. Also (especially Science Fiction): (an arbitrary designation for) an unknown or hypothetical alien planet.

Origin

Late 19th century. From planet + X.

Pronunciation

Planet X

/ˌplanɪt ˈɛks/