Architectures of Exoplanetary Systems. III: Eccentricity and Mutual Inclination Distributions of AMD-stable Planetary Systems

TL;DR

This study uses AMD-stability criteria to model exoplanetary system architectures, revealing how multiplicity influences orbital eccentricities and inclinations, and comparing model predictions with Kepler observations.

Contribution

It demonstrates that a broad distribution of mutual inclinations from AMD-stability limits can reproduce Kepler's observed planetary system multiplicities and orbital properties.

Findings

Higher planet multiplicity correlates with lower eccentricities and inclinations.

Single-planet systems tend to have higher eccentricities than multi-planet systems.

Observed planet size orderings and spacings exceed what detection biases alone can explain.

Abstract

The angular momentum deficit (AMD) of a planetary system is a measure of its orbital excitation and a predictor of long-term stability. We adopt the AMD-stability criteria to constrain the orbital architectures for exoplanetary systems. Previously, He, Ford, & Ragozzine (2019) (arXiv:1907.07773v2) showed through forward modelling (SysSim) that the observed multiplicity distribution can be well reproduced by two populations consisting of a low and a high mutual inclination component. Here, we show that a broad distribution of mutual inclinations arising from systems at the AMD-stability limit can also match the observed Kepler population. We show that distributing a planetary system's maximum AMD amongst its planets results in a multiplicity-dependent distribution of eccentricities and mutual inclinations. Systems with intrinsically more planets have lower median eccentricities and mutual inclinations, and this trend is well described by power-law functions of the intrinsic planet multiplicity ($n$): $\tilde{\mu}_{e,n} \propto n^{-1.74_{-0.07}^{+0.11}}$ and $\tilde{\mu}_{i,n} \propto n^{-1.73_{-0.08}^{+0.09}}$, where $\tilde{\mu}_{e,n}$ and $\tilde{\mu}_{i,n}$ are the medians of the eccentricity and inclination distributions. We also find that intrinsic single planets have higher eccentricities ($\sigma_{e,1} \sim 0.25$) than multi-planet systems, and that the trends with multiplicity appear in the observed distributions of period-normalized transit duration ratios. We show that the observed preferences for planet size orderings and uniform spacings are more extreme than what can be produced by the detection biases of the Kepler mission alone. Finally, we find that for systems with detected transiting planets between 5 and 10 days, there is another planet with a greater radial velocity signal $\simeq~53\%$ of the time.