Evidence for a Peak at $\sim$0.3 in the Eccentricity Distribution of Typical Super-Jovian Exoplanets

TL;DR

This paper identifies a significant peak at eccentricity 0.3 in the distribution of super-Jovian exoplanets, suggesting a common dynamical history involving moderate eccentricities.

Contribution

It provides the first evidence of a peak at eccentricity 0.3 for super-Jovian exoplanets using a hierarchical modeling approach on RV survey data.

Findings

Peak at eccentricity 0.3 for super-Jovian planets

92% of samples show elevated occurrence at moderate eccentricities

Super-Jupiters are more common with eccentricities around 0.3

Abstract

In this study, we compute completeness-corrected occurrence rates of giant exoplanets as a function of mass, semimajor axis, and eccentricity, using the approximately uniform California Legacy Survey sample of RV-discovered planets published in Rosenthal et al. 2021. We recover the previously-detected rise in occurrence with semimajor axis for both lower- and higher-mass subsets of the population out to $\sim$5 au. When restricting to planets with semimajor axes between 0.1 and 4.5 au (roughly speaking, the "peak" of giant planet occurrence), we find evidence for distinct eccentricity distributions for each of two mass sub-populations. Most strikingly, we observe a peak in the eccentricity distribution of super-Jovian planets (3-20~M$_{\rm J}$) at 0.3, which is apparent using two different parameterizations of the eccentricity distribution model. A hierarchical histogram model reveals that $\sim$92% of posterior samples indicate an elevated occurrence rate of super-Jupiters with modest eccentricities (0.2-0.4) compared to lower or higher eccentricities (i.e. evidence for a moderate eccentricity "peak"), and 99% of samples indicate super-Jupiters with modest eccentricities are more common than those with lower eccentricities (i.e. evidence that moderate eccentricities are more common than low eccentricities). We use a truncated Gaussian model fit to pinpoint the location of the super-Jupiter eccentricity peak with more precision, finding a maximum a posterior (MAP) peak location of $e=0.3$. This low but elevated characteristic eccentricity could be the result of dynamically hot histories, perhaps involving a giant impacts phase. All analysis code for this project is publicly available on Zenodo (https://zenodo.org/records/18089157) and GitHub (github.com/sblunt/eccentricities).