Duck Dodgers in the 24-1/2 Century
When Pluto was later discovered in 1930, Lowell's hypothesis seemed to have some validity, until Pluto was shown to be too small to have a significant effect on the gas giant's orbit. This prompted still more searching, this time for the 10th planet. The search for Planet X continued all the way up to 1990 before Voyager 2 discovered that the reason the gas giants had unexplained orbits was merely due to an overestimation of Neptune's mass.
Planet X came about due to an overestimation of Neptune's mass
Nowadays, Planet X has been largely forgotten. It does not exist as we once thought it might, and Pluto isn't even a planet anymore. Some scientists still use Planet X as a term to denote any planet yet to be discovered, and a few have even revisited the hypothesis as a way of explaining some anomalies in the outer solar system. There may also be some questionable theories surrounding Planet X and the hypothetical dwarf star "Nemesis", but I won't go into that right now.