Pluto was discovered, in part, out of the general search for Planet X. After the discovery of Neptune, astronomers noticed that it was not appearing where the laws of physics, which accurately described the orbits of the other planets, predicted. Uranus tended to be slightly off as well. This prompted the theory that something beyond Neptune was “tugging at it”. The discovery of Pluto was thought to have provided the answer, until astronomers quickly figured out that Pluto is not nearly massive enough to be the culprit. So the search for something larger beyond Neptune continues. I find the thought that the culprit may not be a planet but a brown dwarf or other burned out star orbiting the sun (or perhaps the other way around) very intriguing.
Ditto. There’s a school of thought that says that the Neptunian ephemeris can be better explained by dropping out a small number of observations, making the Planet X problem (quote) a psychological one (unquote). :’) That’s not my view, but hey, hardly anyone listens to me. ;’)