Direct imaging characterization of cool gaseous planets

Michiel Min; Jo Barstow; Laura C. Mayorga; Hannah Wakeford; Jason Wang; Renyu Hu; Beth Biller; José A. Caballero; Ludmila Carone; Sarah Casewell; Katy L. Chubb; Mario Damiano; Siddharth Gandhi; Antonio García Muñoz; Christiane Helling; Finnegan Keller; Nataliea Lowson; Evert Nasedkin; Ryan MacDonald; Jean-Baptiste Ruffio; Evgenya Shkolnik; Christopher C. Stark

arXiv · 2512.13766

Direct imaging characterization of cool gaseous planets

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Abstract

Cool gas giant exoplanets, particularly those with properties similar to those of Jupiter and Saturn, remain poorly characterized due to current observational limitations. This white paper outlines the transformative science case for the Habitable Worlds Observatory (HWO) to directly image and spectroscopically characterize a broad range of gaseous exoplanets with effective temperatures below 400 K. The study focuses on determining key atmospheric properties, including molecular composition, cloud and haze characteristics, and temperature structure, across planets of varying sizes and orbital separations. Leveraging reflected light spectroscopy and polarimetry, HWO will enable comparative planetology of cool gas giants orbiting both solar-type and M-dwarf stars, bridging the observational gap between hot exoplanets and Solar System giants. We present observational requirements and survey strategies necessary to uncover correlations between atmospheric properties and planetary or stellar parameters. This effort will establish critical constraints on planetary formation, cloud microphysics, and the role of photochemistry under diverse irradiation conditions. The unique capabilities of HWO will make it the first facility capable of characterizing true exo-Jupiters in reflected light, thus offering an unprecedented opportunity to place the Solar System in a broader galactic context.

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Michiel Min, Jo Barstow, Laura C. Mayorga, Hannah Wakeford, Jason Wang, Renyu Hu, Beth Biller, José A. Caballero, Ludmila Carone, Sarah Casewell, Katy L. Chubb, Mario Damiano, Siddharth Gandhi, Antonio García Muñoz, Christiane Helling, Finnegan Keller, Nataliea Lowson, Evert Nasedkin, Ryan MacDonald, Jean-Baptiste Ruffio, Evgenya Shkolnik, Christopher C. Stark. 2025-12-15. Direct imaging characterization of cool gaseous planets. https://arxiv.org/abs/2512.13766

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