At the outskirts of the solar system, beyond the orbit of Neptune, lies an expansive field of icy debris known as the Kuiper belt. The orbits of the individual asteroid-like bodies within the Kuiper belt trace out highly elongated elliptical paths, and require hundreds to thousands of years to complete a single revolution around the Sun. Although the majority of the Kuiper belt’s dynamical structure can be understood within the framework of the knowneight-planet solar system, bodies with orbital periods longer than ~4,000 years exhibit a peculiar orbital alignment that eludes explanation.

In Batygin & Brown (2016a), we showed that the observed clustering of Kuiper belt orbits can be maintained by a distant, eccentric, Neptune-like planet, whose orbit lies in approximately the same plane as those of the distant Kuiper belt objects, but is anti-aligned with respect to the perihelia of the minor bodies. The particular mode of dynamical coupling responsible for the observed clustering is exceedingly intricate, and involves a complex interplay between mean motion resonances and secular interactions (Batygin & Morbidelli 2017).

In addition to accounting for the observed grouping of trajectories, the existence of such a planet naturally explains other, seemingly unrelated dynamical characteristics of the solar system. Namely, the origins of dynamically detached Sedna-type orbits, the generation of highly inclined (and retrograde) trans-Neptunian objects (Batygin & Brown 2016b) as well as the non-zero obliquity of the Sun (Bailey et al 2016) are all seamlessly reproduced by the gravitational influence of Planet Nine. The observational search for Planet Nine (Brown & Batygin 2016) is now ongoing.