Read what Planet X is and how it affects other planets.
The term Planet X is often applied to any search for an unknown planet.
Astronomers have observed perturbations, or disturbances, in the orbits of Uranus and Neptune since the discoveries of both planets. They speculated that Uranus and Neptune were being influenced by the gravity of another celestial body.
Pluto, discovered in 1930, does not appear to be large enough to cause these disturbances. The existence of another planet, known as Planet X, orbiting beyond Pluto, was proposed.
Unmanned space probes turned up no evidence of such an object and the hunt for Planet X was, for the most part, abandoned by the astronomical community.