Wiktionary
alt. A hypothetical planet sharing an orbit with the earth, but on the opposite side of the sun. n. A hypothetical planet sharing an orbit with the earth, but on the opposite side of the sun.
Wikipedia
The Counter-Earth is a hypothetical body of the Solar system 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 an unseen "Central Fire" (distinct from the Sun which also revolves around it). The Greek word Antichthon means "Counter-Earth."
In modern times a hypothetical planet always on the other side of the Sun from the Earth has been called a "Counter-Earth", and has been a recurring theme in fiction, science fiction and UFO claims.
Counter-Earth is the name of a fictional planet in Marvel Comics. There have been four versions of the hypothetical planet known as Counter-Earth, each one a near-duplicate of Earth.
Usage examples of "counter-earth".
From there, they spread out in both directions on the plane of probabilities, defeating one counter-Earth after another.