Planet X

I first came across mention of Planet X when I was watching an old Daffy Duck cartoon with my father when I was very young. The idea intrigued me then, and I was quick to learn the truth about this mysterious planet. After Neptune was discovered in 1846, there was an awful lot of speculation that other planets might exist beyond its orbit. This speculation was later fueled by the observation that the gas giants Neptune and Uranus had strange orbits that were not explainable given what astronomers knew at the time. In the 1890's a man by the name of Percival Lowell proposed that a planet, which he named Planet X, was responsible for the strange orbit of these gas giants. The search for the 9th planet was officially on.

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.