MIT Technology Review has a brief article on the hunt for Planet 9:
But a much smaller class, known as the detached population, have entirely different orbital properties. Some have retrograde orbits; others are highly eccentric or on paths that are dramatically inclined with respect to the sun’s plane.
Neptune cannot account for this kind of behavior. So the hypothesis that Batygin and others are exploring is that some other massive object—call it Planet 9—must be responsible.
You can read the paper itself on the arXiv. I still find it hard to believe that we can access academic papers directly on our phones.