One of the other theories for Planet X I believe has been debunked as absense of evidence. There are gaps in the documented bodies orbiting the sun that could imply an object clearing orbits, but they were dismissed instead as sampling errors - there are parts of the sky that are easier to catalog than others, and so of course we have cataloged the easy parts more thoroughly. We need observation stations in a sun orbit to see the parts we can’t see easily from an earth orbit.