Resonances of Multiple Exoplanets and Implications for Their Formation

TL;DR

This study uses numerical simulations to explore how the observed resonant configurations of multiple exoplanets constrain their formation environment, particularly the stage of disk evolution during planet assembly.

Contribution

It demonstrates that the observed mean motion resonances are indicative of the disk conditions at the time of planet formation, especially highlighting differences between massive and low-mass planets.

Findings

Massive planets tend to end up in 2:1 MMR regardless of disk mass.

Low-mass planets can only have MMRs larger than 4:3 in low-mass disks.

The observed architectures suggest low-mass planets form late in disk evolution.

Abstract

Among $\sim 160$ of the multiple exoplanetary systems confirmed, about $30\%$ of them have neighboring pairs with a period ratio $\leq 2$. A significant fraction of these pairs are around mean motion resonance (MMR), more interestingly, peak around 2:1 and 3:2, with a clear absence of more closely packed MMRs with period ratios less than 4:3, regardless of planet masses. Here we report numerical simulations demonstrating that such MMR behavior places important constraints on the disk evolution stage out of which the observed planets formed. Multiple massive planets (with mass $\geq 0.8$ $M_{\rm Jup}$) tend to end up with a 2:1 MMR mostly independent of the disk masses but low-mass planets (with mass $\leq 30$ $M_{\oplus}$) can have MMRs larger than 4:3 only when the disk mass is quite small, suggesting that the observed dynamical architecture of most low-mass-planet pairs was established late in the disk evolution stage, just before it was dispersed completely.