Do Planetesimal Interactions Explain the Formation of Near-Resonant & Compact Exoplanet Systems

Planetesimal Interactions Can Explain the Mysterious Period Ratios of Small Near-Resonant Planets

Authors:

Chatterjee et al

Abstract:

An intriguing trend among the Kepler multi-planet systems is a clear over abundance of planet pairs with period ratios just wide of several mean motion resonances (MMR) and a dearth of systems just narrow of them. Traditional planet formation models such as gas-disk migration or planet-planet scattering are at odds with these observations. This is also in dramatic contrast to the period ratio distribution of multi-planet systems discovered via radial velocity (RV) surveys, where near-resonant planet pairs tend to pile up near exact integer ratio of orbital periods such as 2:1, as expected from gas-disk migration. We propose that gas-disk migration traps planets in a MMR. After gas dispersal, orbits of these trapped planets are altered through interaction with a residual planetesimal disk. We study the effects of planetesimal disk interactions on planet pairs trapped in 2:1 MMR using planets of mass typical of the KPCs and explore large ranges for the mass, and density profile of the planetesimal disk. We find that planet-planetesimal disk interactions naturally create the observed asymmetry in period ratio distribution for large ranges of planetesimal disk and planet properties. If the planetesimal disk mass is above a threshold of 0.2x the planet mass, these interactions typically disrupt MMR. Then the planets migrate in such a way that the ratio of their final orbital periods is slightly higher than the integer ratio corresponding to the initial MMR. Below this threshold these interactions typically cannot disrupt the resonance and the period ratio stays close to the integer ratio. The threshold explains why the more massive planet pairs found by RV surveys are still in resonance. These interactions can make significant changes in the planets' atmospheric and surface properties, especially for small planets typical of the KPCs, which may be observable in the near future.