Breaking the chains: hot super-Earth systems from migration and disruption of compact resonant chains

Abstract

"Hot super-Earths" (or "Mini-Neptunes") between 1 and 4 times Earth's size with period shorter than 100 days orbit 30-50\% of Sun-like type stars. Their orbital configuration -- measured as the period ratio distribution of adjacent planets in multi-planet systems -- is a strong constraint for formation models. Here we use N-body simulations with synthetic forces from an underlying evolving gaseous disk to model the formation and long-term dynamical evolution of super-Earth systems. While the gas disk is present, planetary embryos grow and migrate inward to form a resonant chain anchored at the inner edge of the disk. These resonant chains are far more compact than the observed super-Earth systems. Once the gas dissipates resonant chains may become dynamically unstable. They undergo a phase of giant impacts that spreads the systems out. Disk turbulence has no measurable effect on the outcome. Our simulations match observations if a small fraction of resonant chains remain stable, while most super-Earths undergo a late dynamical instability. Our statistical analysis restricts the contribution of stable systems to less than $25\%$. Our results also suggest that the large fraction of observed single planet systems does not necessarily imply any dichotomy in the architecture of planetary systems. Finally, we use the low abundance of resonances in Kepler data to argue that, in reality, the survival of resonant chains happens likely only in $\sim 5\%$ of the cases. This leads to a mystery: in our simulations only 50-60\% of resonant chains became unstable whereas at least 75\% (and probably 90-95\%) must be unstable to match observations.