The Library of Exoplanet Atmospheric Composition Measurements: Population Level Trends in Exoplanet Composition with ExoComp

TL;DR

This paper introduces ExoComp, an open-source toolkit and a comprehensive library of exoplanet atmospheric measurements, revealing population-level trends in composition, metallicity, and C/O ratios among gas giants, with implications for planetary formation theories.

Contribution

The paper presents a standardized toolkit, ExoComp, and a curated library of exoplanet atmospheric data, enabling consistent comparison and analysis of population-level compositional trends.

Findings

Exoplanets show higher metallicity compared to T-dwarfs and stars.

C/O ratios are constrained between 0 and 1 across measurements.

Transit spectroscopy yields systematically lower C/O ratios than eclipse and direct methods.

Abstract

The present-day bulk elemental composition of an exoplanet can provide insight into a planet's formation and evolutionary history. Such information is now being measured for dozens of planets with state-of-the-art facilities using Bayesian atmosphere retrievals. We collect measurements of exoplanet composition of gas giants into a Library of Exoplanet Atmospheric Composition Measurements for comparison on a population level. We develop an open-source toolkit, ExoComp, to standardize between solar abundance, metallicity, and C/O ratio definitions. We find a systematic enhancement in the metallicity of exoplanets compared to T-dwarf and stellar populations, a strict bound in C/O between 0 and 1, and statistically significant differences between measurements from direct, eclipse, and transmission spectroscopy. In particular, the transit spectroscopy population exhibits a systematically lower C/O ratio compared to planets observed with eclipse and direct spectroscopy. While such differences may be astrophysical signals, we discuss many of the challenges and subtleties of such a comparison. We characterize the mass-metallicity trend, finding a slope consistent between planets measured in transit versus eclipse, but offset in metallicity. Compared to the Solar System and constraints from interior modeling, gas giant atmospheres appear to exhibit a steeper mass-metallicity trend. We hope that the tools available in ExoComp and the data in the Library of Exoplanet Atmospheric Composition Measurements can enhance the science return of the wide-array of space- and ground-based exoplanet science being undertaken by the community.