Formation of planetary systems from pebble accretion and migration II: Hot super-Earth systems from breaking compact resonant chains

At least 30% of the FGK-type stars host "hot Super-Earths" with sizes between 1 and 4 Earth radii and orbital periods of less than 100 days. Here we use N-body simulations that simultaneously model gas-assisted pebble accretion and disk-planet tidal interaction to study the formation of hot super-Earths systems. Our results show that the integrated pebble mass reservoir primarily controls the system fate. A simple factor of ̃2 difference in the pebble flux bifurcates the final outcome of our simulations between systems of hot super-Earths or hot- Neptunes (≤20 MEarth) and systems containing multiple massive cores able to become gas giants (≥20 MEarth). Simulations with low to moderate pebble fluxes grow multiple super-Earths that migrate inward and pile up at the disk's inner edge in long resonant chains. We follow the long-term dynamical evolution of these systems and use the period ratio distribution of observed planet-pairs to constrain our model. We find that up to ̃95% of the resonant chains become dynamically unstable after the gas disk dispersal. Supporting previous studies, our simulations match observations if we combine a fraction of unstable and stable systems (e.g. 95% unstable plus 5% stable). Our results also reinforce the claim that the Kepler dichotomy is an observational artifact. Finally, our results predict that in every hot super-Earth system at least some hot super-Earths should be ice-rich, if not most of them. If observations instead find a lack of ice-rich super- Earths it may suggest that planetesimal formation is far more efficient than expected well inside the snowline, or perhaps that pebbles in the inner refractory disk are as large as in the outer icy disk. Indeed, this would favor rapid growth in the inner disk, although it is at odds with the inferred conditions in the early solar system. This talk comes as the 2nd part of a series of papers, where the 1st talk will be presented by Dr. S. Jacobson about the formation of terrestrial planets and rocky super-Earths. The 3rd talk will be presented by Dr. B. Bitsch about the formation of gas giant planets.