Session X15: Dynamics of Planetary Systems

Planet 9 From Outer Space

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 known eight-planet solar system, bodies with orbital periods longer than about 4,000 years exhibit a peculiar orbital alignment that eludes explanation. What sculpts this alignment and how is it preserved? In this talk, I will argue 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 those of the small bodies. In addition to accounting for the observed grouping of orbits, the existence of such a planet naturally explains other, seemingly unrelated dynamical features of the solar system.   

Multi-Planet Dynamics as a Tracer of Planet Formation

Gravitational interactions between neighboring planets reshape the dynamical structure of some and perhaps most planetary systems. By redistributing orbits, they obscure the locations where planets formed in individual systems. However, when treated as an ensemble, systems that experienced planet-planet interactions provide clues to the parameters that cause planet formation to differ around different stars. I will discuss differences between giant planet orbits in high- and low-metallicity systems, suggest a framework for explaining these differences in the context of planet formation, and comment on the conditions that lead to large-scale dynamical upheavals, with particular reference to the currently-stable system HR 8799. Our own solar system, though currently dynamically unexcited, may have experienced an upheaval in the past. I will end by demonstrating that the population of Kuiper belt objects in the distant 5:2 resonance with Neptune challenges standard models of the dynamical history of the outer solar system.   

Strongly Interacting Planetary Systems

Both ground-based Doppler surveys and NASA's Kepler mission have discovered a diversity of planetary system architectures that challenge theories of planet formation. Systems of tightly-packed or near-resonant planets are particularly useful for constraining theories of orbital migration and the excitation of orbital eccentricities and inclinations. In particular, transit timing variations (TTVs) provide a powerful tool to characterize the masses and orbits of dozens of small planets, including many planets at orbital periods beyond the reach of both current Doppler surveys and photoevaporation-induced atmospheric loss. Dynamical modeling of these systems has identified some "supper-puffy" planets, i.e., low mass planets with surprisingly large radii and low densities. I will describe a few particularly interesting planetary systems and discuss the implications for the formation of planets ranging from gaseous super-Earth-size planets to rocky planets the size of Mars.