Hypothetical planets are planets and other heavenly bodies that have been suggested as possibly existing (or have been believed to exist), but have never been proven to actually exist. This includes bodies that have later been proven not to exist.
Planets which would have a direct influence on Earth:
- Counter-Earth: An Earth-like planet on Earth's orbit but on the other side of the Sun (hypothesized by the Pythagoreans; used by John Norman as the setting for his Gor novels)
- Theia, a hypothetical planet which was destroyed in a collision with Earth according to the giant impact theory
- Nibir/Marduk and Tiamet, two planets from Sumerian mythology which supposedly collided to form Earth (a theory which most scientists find dubious)
- Planet X (a tenth planet beyond Pluto)
- Vulcan, inside the orbit of Mercury (like Planet X, it was proposed to explain orbital peculiarities)
Giant impact theory, Tom Van Flandern