A Precise Metallicity and Carbon-to-Oxygen Ratio for a Warm Giant Exoplanet from its Panchromatic JWST Emission Spectrum

WASP-80 b, a warm sub-Jovian (equilibrium temperature ~820 K, 0.5 Jupiter masses), presents an opportunity to characterize a rare gas giant exoplanet around a low-mass star.

In addition, its moderate temperature enables its atmosphere to host a range of carbon and oxygen species (H₂O, CH₄, CO, CO₂, NH₃).

In this paper, we present a panchromatic emission spectrum of WASP-80 b, the first gas giant around a late K/early M-dwarf star and the coolest planet for which the James Webb Space Telescope has obtained a complete emission spectrum spanning 2.4-12 μm, including NIRCam F322W2 (2.4-4 μm) and F444W (4-5 μm), and MIRI LRS (5-12 μm).

We report confident detections of H₂O, CH₄, CO, and CO₂, and a tentative detection of NH₃. We estimate WASP-80 b’s atmospheric metallicity and carbon-to-oxygen ratio and compare them with estimates for other gas giants.

Despite the relative rarity of giant planets around low-mass stars, we find that WASP-80 b’s composition is consistent with other hot gas giants, suggesting that the formation pathway of WASP-80 b may not be dissimilar from hot gas giants around higher-mass stars.

Transiting giant planets around low-mass stars that have been observed, or will be observed, with JWST through Observation Cycle 3. We define these “low stellar mass Jovians” as planets with stellar mass < 0.6M_Sun and planet mass > 0.3M_Jupiter and shade that region in green. JWST targets with masses just outside of those ranges are also plotted in green. Grey points show all other known transiting planets from the NASA Exoplanet Archive. — astro-ph.EP

Lindsey S. Wiser, Taylor J. Bell, Michael R. Line, Everett Schlawin, Thomas G. Beatty, Luis Welbanks, Thomas P. Greene, Vivien Parmentier, Matthew M. Murphy, Jonathan J. Fortney, Kenny Arnold, Nishil Mehta, Kazumasa Ohno, Sagnick Mukherjee

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2506.01800 [astro-ph.EP] (or arXiv:2506.01800v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2506.01800
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From: Lindsey Wiser
[v1] Mon, 2 Jun 2025 15:41:33 UTC (4,917 KB)
https://arxiv.org/abs/2506.01800
Astrobiology,