Abstract

As the population of known exoplanets grows, patterns are emerging in the multiplicities of planetary systems. Hot Jupiters, giant planets on orbits of a few days, rarely seem to have close companions, but often have distant companions (other planets or binary stars beyond 1au). Meanwhile, Kepler results show that super-Earths and Neptunes within 0.5au are (1) very common (orbiting 50% of stars), (2) often in multi-planet systems, but (3) that there seems to be a separate population of single super-Earths distinct from the multiple systems.

In this talk I discuss planetary multiplicities from a dynamical perspective. I first review proposed mechanisms for the migration of Hot Jupiters, and argue that their lack of close companion planets is consistent with the “dynamical migration” mechanism of excitation of a high-eccentricity orbit followed by tidal circularisation. I then turn my attention to the population of super-Earths and ask whether the single-planet systems can be the descendants of multiples as their numbers are culled by dynamical processes (internal instability, or scattering or other forcing of undetected planets in the outer system), or whether their singledom may date back to the very epoch of planet formation.