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-Neptunianobjects appears to display dynamical phenomena that cannot be explained by interactions with the known eight-planet Solar System alone. Specifically, the observed physical clustering of orbits with semi-major axes in excess of ∼ 250 AU, 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 planet with mass M9 ∼ 5M⊕ on a moderately inclined orbit with a semi-major axis a9 ∼ 400−800 AU and eccentricity e9 ∼0.4−0.6. In this talk, I will review the observational motivation, dynamical constraints, and prospects for the detection of this proposed object known as Planet Nine.
