From the New Scientist:
When potential dwarf planet 2012 VP113 was discovered in March, it joined a handful of unusual rocky objects known to reside beyond the orbit of Pluto. These small objects have curiously aligned orbits, which hints that an unseen planet even further out is influencing their behaviour. Scientists calculated that this world would be about 10 times the mass of Earth and would orbit at roughly 250 times Earth's distance from the sun.
... Some of the smaller objects have very elongated orbits that would take them out to this distance. It is unusual for a large planet to orbit so close to other bodies unless it is dynamically tied to something else, so the researchers suggest that the large planet is itself in resonance with a more massive world at about 250 times the Earth-sun distance – just like the one predicted in the previous work.
Observing these putative planets will be tricky. The smaller bodies are on very elliptical orbits and were only spotted when they ventured closest to the sun. But the big planets would have roughly circular orbits and would be slow moving and dim, making them tough for current telescopes to see. "It's not at all surprising that they haven't been found yet," says Carlos.H/t Instapundit