Planet X

Planet X is a [hypothetical planet]? beyond Pluto. Its existence was argued for on the basis of apparent discrepancies in the orbit of Neptune. This led to the discovery for Pluto, but Pluto's mass was not sufficent to explain Neptune's orbit, so the search continued. However these apparent discrepancies were resolved when the Voyager space probes discovered that Neptune's mass had been misestimated; with Neptune's newly discovered mass taken into account, there was no longer a need for any new planet to explain Neptune's orbit.