Dynamical Evolution of Planetary Systems

Planetary systems can evolve dynamically even after the planets themselves have fully formed, and there is circumstantial evidence that most planetary systems become unstable after the disappearance of the gaseous protoplanetary disk. Theories of planet formation predict that chains of mean motion resonances are the natural outcome of disk-driven planet migration, leading to the pile up of super-Earths resonant chains close to the inner edge of the disk and the formation of fragile chains for distant giant planets. Observations of young systems suggest that they are more often locked in these chains than older ones, which are instead mostly non-resonant. The instabilities thought responsible for this trend can arise intrinsically if the original systems are too closely packed, or be due to external perturbations such as tides, planetesimal scattering, or torques from distant stellar companions. The Solar System was not exceptional in this sense, as the outer giants saw the disruption of a resonant chain; meanwhile, the inner system was likely built through a series of giant impacts between closely packed planetary embryos. Thus, the orbital distributions of planetary systems that is observed today, both solar and extrasolar, can be different from those emerging from formation and assembly processes within the disk, and it is important to consider possible long-term dynamics to connect the two.