The Resonant Remains of Broken Chains from Major and Minor Mergers

Abstract: TESS and Kepler have revealed that practically all close-in sub-Neptunes form in mean-motion resonant chains, most of which unravel on timescales of 100 Myr. Using N-body integrations, we study how planetary collisions from destabilized resonant chains produce the orbital period distribution observed among mature systems, focusing on the resonant fine structures remaining post-instability. In their natal chains, planets near first-order resonances have period ratios just wide of perfect commensurability, driven there by disk migration and eccentricity damping. Sufficiently large resonant libration amplitudes are needed to trigger instability. Ensuing collisions between planets ("major mergers") erode but do not eliminate resonant pairs; surviving pairs show up as narrow "peaks" just wide of commensurability in the histogram of neighboring-planet period ratios. Merger products exhibit a broad range of period ratios, filling the space between relatively closely-separated resonances such as the 5:4, 4:3, and 3:2, but failing to bridge the wider gap between the 3:2 and 2:1 -- a "trough" thus manifests just short of the 2:1 resonance, as observed. Major mergers generate debris which undergoes "minor mergers" with planets, in many cases further widening resonant pairs. With all this dynamical activity, free eccentricities of resonant pairs, and by extension the phases of their transit timing variations, are readily excited. Non-resonant planets, being merger products, are predicted to have higher masses than resonant planets, as observed. At the same time, a small fraction of mergers produce a high-mass tail in the resonant population, also observed.