Planet 9 a conspiracy theory? Who's conspiring against whom there :|

Afaik it was a legit theory since we discovered planet 8 that way and then people tried to use the same method for further planets. Also beyond Mercury there was supposed to be Vulcanus and people reported sightings but nothing added up

Discovery of planet 8 (Wikipedia):

unexpected changes in the orbit of Uranus led Alexis Bouvard to hypothesise that its orbit was subject to gravitational perturbation by an unknown planet. After Bouvard's death, the position of Neptune was mathematically predicted from his observations, independently, by John Couch Adams and Urbain Le Verrier. Neptune was subsequently directly observed with a telescope

And then Mr Einstein had a thing or two to say about those gravitational disturbances being actually relativity and most things clicked into place (but you'll still have a discrepancy between the known spacetime curving and observed orbits because it's hard to know what mass is exactly where in the Kuiper belt etc.). Or something. I'm probably wrong on the details but that's the broad strokes as I remember them

We didn't get planet 9 in school either fwiw but I think it was in magazines or encyclopedia at my grandparents' place that I heard of it