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Wiktionary
counter-earth

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
Counter-Earth

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 (comics)

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.