The planet nine hypothesis

Over the course of the past two decades, observational surveys have unveiled the intricate orbital structure of the Kuiper Belt, a field of icy bodies orbiting the Sun beyond Neptune. In addition to a host of readily-predictable orbital behavior, the emerging census of trans-Neptunian objects displays dynamical phenomena that cannot be accounted for by interactions with the known eight-planet solar system alone. Specifically, explanations for the observed physical clustering of orbits with semi-major axes in excess of ∼250AU, the detachment of perihelia of select Kuiper belt objects from Neptune, as well as the dynamical origin of highly inclined/retrograde long-period orbits remain elusive within the context of the classical view of the solar system. This newly outlined dynamical architecture of the distant solar system points to the existence of a new planet with mass of m_g ∼5−10M⊕, residing on a moderately inclined orbit (i_g∼15−25deg) with semi-major axis a_g∼400−800AU and eccentricity between e_g∼0.2−0.5. This paper reviews the observational motivation, dynamical constraints, and prospects for detection of this proposed object known as Planet Nine.