There's no evidence for the existence of Planet X, despite a NASA space telescope's best efforts to track it down.
The hypothetical world that may or may not be orbiting the sun beyond the orbit of Pluto has inspired many a doomsday theory. In the run-up to the much anticipated "Mayan Doomsday" of Dec. 21, 2012, the marauding Planet X was scheduled to make a inner-solar system dash, sparking gravitational mayhem, triggering civilization-ending solar flares. Some doomsayers held onto the crackpot notion that Planet X could be the fictional planet "Nibiru" that is inhabited by the Annunaki, an alien race hellbent on re-claiming Earth as their own.
15 months later, we all know how that alien invasion went - apparently we won.
Top Doomsday Predictions Gone Bust
All this doomsday nonsense to one side, the hunt for "Planet X" actually has roots in real science. In the mid- to late-19th Century, astronomers were tracking the gravitational perturbations of the gas giant planets in an effort to track down an undiscovered world in the outermost reaches of the solar system - this hypothetical massive planet was dubbed "Planet X." However, this fascinating trail of discovery ended at the discovery of tiny Pluto in 1930. Lacking the gravitational oomph to explain the gravitational perturbations, it turned out that Pluto wasn't the Planet X astronomers thought it would be. After the realization that the gravitational perturbations observed were more likely observational error, Planet X became a story of legend.