2026AJ....171...31L

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 tool kit, 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.