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

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.

Joshua D. Lothringer, Nataliea Lowson, Guangwei Fu

Comments: 27 pages, 10 figures, 6 tables. Table 4 available as MRT in source. ExoComp available at this https URL. Accepted for publication to AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2510.26785 [astro-ph.EP] (or arXiv:2510.26785v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2510.26785
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Submission history
From: Joshua Lothringer
[v1] Thu, 30 Oct 2025 17:57:24 UTC (889 KB)
https://arxiv.org/abs/2510.26785
Astrobiology, Astrochemistry, Astrogeology,