Short-Period Small Planets with High Mutual Inclinations are more Common around Metal-Rich Stars

TL;DR

This study finds that short-period small planets around metal-rich stars tend to have higher and more diverse mutual inclinations, indicating dynamically hotter inner planetary systems compared to metal-poor stars.

Contribution

It reveals a correlation between stellar metallicity and mutual inclinations in short-period small planet systems, a relationship not previously well characterized.

Findings

Mutual inclinations are higher around metal-rich stars.

Inner systems around metal-rich stars are dynamically hotter.

The mean mutual inclination for metal-rich systems is ~3.1 degrees.

Abstract

We present a correlation between the stellar metallicities and the mutual inclinations of multi-planet systems hosting short-period small planets (a/Rs<12, Rp<4Re). We analyzed 89 multi-planet systems discovered by Kepler, K2, and TESS, where the innermost planets have periods shorter than 10 days. We found that the mutual inclinations of the innermost two planets are higher and more diverse around metal-rich stars. The mutual inclinations are calculated as the absolute differences between the best-fit inclinations of the innermost two planets from transit modeling, which represent the lower limits of the true mutual inclinations. The mean and variance of the mutual inclination distribution of the metal-rich systems are 3.1+-0.5 and 3.1+-0.4 degrees, while for the metal-poor systems they are 1.3+-0.2 and 1.0+-0.2 degrees. This finding suggests that inner planetary systems around metal-rich stars are dynamically hotter. We summarized the theories that could plausibly explain this correlation, including the influence of giant planets, higher solid densities in protoplanetary disks around metal-rich stars, or secular chaos coupled with an excess of angular momentum deficits. Planet formation and population synthesis models tracking the mutual inclination evolution would be essential to fully understand this correlation.