Counter-Earth

The Counter-Earth is a hypothetical body of the Solar System that orbits on the other side of the Solar System from Earth. A Counter-Earth or Antichthon (Greek: Ἀντίχθων) was hypothesized by the pre-Socratic Greek philosopher Philolaus (c. 470 – c. 385 BC) to support his non-geocentric cosmology, in which all objects in the universe revolve around a "Central Fire" (unseen from Earth and distinct from the Sun which also revolves around it).

In modern times a hypothetical planet always on the other side of the Sun from Earth has been called a "Counter-Earth", and has been a recurring theme in UFO claims, as well as in fiction (particularly science fiction).