There's a list of 'weird' things that need to be explained. A big 9th planet orbiting within a fairly tight range of parameters would explain many things on tha

There's a list of 'weird' things that need to be explained. A big 9th planet orbiting within a fairly tight range of parameters would explain many things on that list. A 10th or 11th planet might also explain it, but not as well, and it's a more complicated answer to have 2 or 3 new planets instead of one. Occam's razor: the simplest explanation is the most likely.