Antichthon: a mythical twin of Earth, in our same orbit around the sun, always exactly opposite the sun to our position

The idea of Antichthon, or "Counter-Earth," as it's also known, dates back to a philosopher in Ancient Greece, who was trying to make sense of the movement of celestial bodies at a time when the world still believed Earth to be located at the center of the universe.

It was nonsense, of course, but that didn't stop science fiction from jumping on the bandwagon and providing multiple representations of Counter-Earth across all mediums of fiction. Noteworthy examples include the The Gor Saga, a series of novels (38 of them!) by John Norman; Marvel Comics' "Halfworld"; episodes of Space: 1999 and Doctor Who; the 1969 film Doppelgänger; and the 2011 movie Another Earth.

Both of those last two added the juicy complication that not only do we have a twin planet orbiting opposite us, but it exists in a timeline exactly parallel to ours, both unfolding synchronically.

Oh, and it's pronounced "an-TICK-thon."

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