The early Solar System was a turbulent and chaotic place with icy material hurled far from the Sun, becoming the Oort Cloud. Larger objects and planets were probably hurled into the Oort Cloud too, and some might have been kicked out of the Solar System entirely. If a similar situation happened in other star systems, planets could lurk out in the Oort Cloud.
This hypothesis is old! Pluto was found because there were perturbations in Neptune’s orbit that couldn’t be explained at the time (hell, that’s how they found Neptune too). https://en.wikipedia.org/wiki/Planets_beyond_Neptune#Planet_X
But pluto was never large enough to answer for those perturbations. So the search continues. The hypothesis gets renamed now and again, and constraints get added. Calling it “Planet Nine” seems trendy these days ;)