http://www.caltech.edu/news/caltech-res ... anet-49523
It's all theory and speculation at this point, and also a subtle dig at Pluto's somewhat recent (2006-ish?) demotion to dwarf-planet. Their argument is compelling, at least, and is based on the inclination and direction of several large-ish Kuiper belt objects' orbits (Sedna being one of them). Obviously, we won't know until we make direct observations to determine if this world exists, and perhaps there's another explanation (besides a 9th planet), but it's still interesting.
There's a related article on the New York Times that's worth reading.
Of course, this doesn't mean such a planet exists, but the theory is that something is disrupting the orbits of these objects--something pretty big. Queue conspiracist panic over Nemesis, Nibiru, Planet X, or whatever you want to call it!