http://www.scientificamerican.com/ar...e-planet-nine/
I don't think I saw a thread on this, but this report came out earlier this week about the hypothetical existence of a 9th planet (sorry Pluto). They are basing the theory on the orbits of Kupier Belt objects and are guessing it is about 10 times the mass of Earth, but its orbit is way out there. Now they just have to actually find it!