Chains of Planets in Mean Motion Resonances Arising from Oligarchic Growth

TL;DR

This study explores how in situ formation of close-in super-Earths and mini-Neptunes can naturally lead to resonant chains through eccentricity damping during the gas disk phase, matching observed Kepler systems.

Contribution

It demonstrates that in situ formation with gas disk dissipation can produce resonant chains similar to observed exoplanet systems, a novel insight into planet formation.

Findings

Resonant chains can form during the gas disk stage without subsequent giant impacts.

Simulated systems match observed period ratios and resonant angles of Kepler systems.

Resonant locking is facilitated by eccentricity damping during planet formation.

Abstract

Exoplanet systems with multiple planets in mean motion resonances have often been hailed as a signpost of disk driven migration. Resonant chains like Kepler-223 and Kepler-80 consist of a trio of planets with the three-body resonant angle librating and/or with a two-body resonant angle librating for each pair. Here we investigate whether close-in super-Earths and mini-Neptunes forming in situ can lock into resonant chains due to dissipation from a depleted gas disk. We simulate the giant impact phase of planet formation, including eccentricity damping from a gaseous disk, followed by subsequent dynamical evolution over tens of millions of years. In a fraction of simulated systems, we find that planets naturally lock into resonant chains. These planets achieve a chain of near-integer period ratios during the gas disk stage, experience eccentricity damping that captures them into resonance, stay in resonance as the gas disk dissipates, and avoid subsequent giant impacts, eccentricity excitation, and chaotic diffusion that would dislodge the planets from resonance. Disk conditions that enable planets to complete their formation during the gas disk stage enable those planets to achieve tight period ratios <= 2 and, if they happen to be near integer period ratios, lock into resonance. Using the weighting of different disk conditions deduced by MacDonald et al. (2020) and forward modeling Kepler selection effects, we find that our simulations of in situ formation via oligarchic growth lead to a rate of observable trios with integer period ratios and librating resonant angles comparable to observed Kepler systems.