are planets whose existence has not been verified but are based
on scientific observations and theories of the universe. There
have been six major hypothetical planets – Counter-Earth, Fifth
Planet (Phaeton/Planet V), Planet X, Theia and Vulcan.
Counter-Earth, Phaeton and Planet X have been discounted by
updates in scientific knowledge but it is still unknown whether
Planet V and Theia existed or not.
Counter-Earth was proposed by the philosopher Philolaus in the
fourth century BC. It was believed a planet like Earth but
located on the opposite side of the Earth's orbit was required
to balance the Earth. Since the discovery of gravity and our
ability to view the area from space the idea of Counter-Earth
has been abandoned.