I'm also curious whether they ripped off Counter-Earth from Marvel Comics or the other way around--although I'd bet money that a Planet X on the other side of the sun but in Earth orbit is an older idea than either (if a terrible one).
Somewhat, yes.
 "Antichthon"--the planet opposite Earth--was first imagined up by Philolaus, Pythagorean philosopher of the 5th century B.C.E. Phil was convinced that geocentrism was incorrect and that the planet rotated around something (a central fire which, for some reason, wasn't the sun), but to balance things out in the geometry-based cosmology of the time, needed an object of equal dimension and mass on the other side of the central fire. The extra planet might seem odd to us now, but the non-geocentric solar system it enabled was a breakthrough that wouldn't be improved upon until Kopernikus & Co. two thousand years later.Fictitiously yours, Trent Roman
				
