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

TL;DR

This work addresses the challenge of comparing exoplanet atmospheric compositions retrieved with diverse methods by introducing ExoComp, a Python toolkit that standardizes solar abundance references, metallicity definitions, and C/O parameterizations, and by assembling LExACoM, a homogenized library of 66 measurements for 47 planets from JWST and 8 m-class facilities. Using ExoComp to convert metallicities and C/O ratios to consistent elemental abundances, the authors reveal a population-wide metallicity enrichment relative to both T-dwarfs and host stars, and a C/O distribution largely between 0 and 1 with geometry-driven biases. They quantify a clear mass-metallicity trend, with a steeper atmospheric trend than interior-model predictions, and find little evidence for strong correlations with temperature or stellar properties, suggesting complex formation and mixing histories. The toolkit and library provide a framework for reproducible, population-level exoplanet chemistry studies and highlight the need for standardized abundance reporting in future retrieval analyses.

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.

Figures (10)

• Figure 1: Measured atmospheric abundances in WASP-17b from louie:2025 (black points with error bars) compared to the best-fit chemical equilibrium solution at [M/H] = 0.26, C/O = 0.25, and R/V= $-1.51$ (blue X's). Upper-limits from Na and CO are represented by a one-sided error bar.
• Figure 2: Measured atmospheric abundances in WASP-178b from lothringer:2025 (transparent grey points representing posterior range) compared to the best-fit chemical equilibrium solution at [M/H] = 0.70, C/O = 0.002, and R/V of $-1.33$ (blue X's). We assume a temperature of 3300 K and pressure of 0.001 bar for the chemical equilibrium fit, consistent with the retrieved isothermal temperature structure.
• Figure 3: C/O ratio as a function of metallicity [M/H] as given in Table \ref{['tab:ExoCompTable']}. Planets observed with direct spectroscopy are shown as blue circles, planets observed with eclipse spectroscopy as magenta squares, and planets observed with transit spectroscopy as gold diamonds. Ultra-hot Jupiters are labeled as stars.
• Figure 4: Median values of the O/H (left, $\log_{10}$ parts per trillion) and C/O (right) shown for different sub-samples. The outer-most errorbar shows the standard deviation of measurements from that sample, while the inner errorbar represents the standard deviation of the mean for reference. Also plotted as horizontal black lines are O/H and C/O values from different stellar abundance definitions.
• Figure 5: Metallicity as a function of equilibrium temperature for planets in the eclipse (magenta squares) and transmission (gold diamonds) spectroscopy samples. Ultra-hot Jupiters are labeled as stars.