“One explanation is the presence of an unseen planet, probably smaller than the Earth and probably bigger than Mercury, orbiting in the deep outer solar system…This paper is not a discovery of a planet, but it’s certainly the discovery of a puzzle for which a planet is a likely solution.”
-Statement by lead author Amir Siraj, an astrophysicist and a doctoral candidate in the department of astrophysical sciences at Princeton University, as quoted by CNN News. The presence of a new, distant planet attempts to address the tilted orbits of some distant objects in the Kuiper Belt. The issue is discussed in a recent paper by Siraj and his fellow authors titled Measuring the Mean Plane of the Distant Kuiper Belt and found in the journal Monthly Notices of the Royal Astronomical Society: Letters.
Our Awesome Universe 