Near-circular orbits for planets around M/K-type stars with Earth-like sizes and instellations

TL;DR

This study uses transit data to estimate the orbital eccentricities of Earth-sized exoplanets around M/K-type stars, finding most have nearly circular orbits, suggesting smooth migration histories.

Contribution

It introduces a hierarchical Bayesian approach to infer eccentricity distributions from transit durations for a specific exoplanet sample.

Findings

Most planets have near-circular orbits with mean eccentricity around 0.06.

The eccentricity distribution suggests these planets formed via smooth disk migration.

One planet shows moderate eccentricity, indicating possible dynamical differences.

Abstract

Recent advances have enabled the discovery of a population of potentially Earth-like planets, yet their orbital eccentricity, which governs their climate and provides clues about their origin and dynamical history, is still largely unconstrained. We identify a sample of 17 transiting exoplanets around late-type stars with similar radii and irradiation to that of Earth and use the "photoeccentric effect" - which exploits transit durations - to infer their eccentricity distribution via hierarchical Bayesian modelling. Our analysis establishes that these worlds further resemble Earth in that their eccentricities are nearly circular (mean eccentricity $=0.060_{-0.028}^{+0.040}$ and $\leq0.15$), with the exception of one outlier of moderate eccentricity. The results hint at a subset population of dynamically warmer Earths, but this requires a larger sample to statistically confirm. The planets in our sample are thus largely subject to minimal eccentricity-induced seasonal variability and are consistent with emerging via smooth disk migration rather than violent planet-planet scattering.