High atmospheric metal enrichment for a Saturn-mass planet

Jacob L. Bean (), Qiao Xue, Prune C. August, Jonathan Lunine, Michael Zhang, Daniel Thorngren, Shang-Min Tsai, Keivan G. Stassun, Everett Schlawin, Eva-Maria Ahrer, Jegug Ih and Megan Mansfield
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Jacob L. Bean: University of Chicago
Qiao Xue: University of Chicago
Prune C. August: University of Chicago
Jonathan Lunine: Cornell University
Michael Zhang: University of Chicago
Daniel Thorngren: Johns Hopkins University
Shang-Min Tsai: University of California
Keivan G. Stassun: Vanderbilt University
Everett Schlawin: University of Arizona
Eva-Maria Ahrer: University of Warwick
Jegug Ih: University of Maryland
Megan Mansfield: University of Arizona

Nature, 2023, vol. 618, issue 7963, 43-46

Abstract: Abstract Atmospheric metal enrichment (that is, elements heavier than helium, also called ‘metallicity’) is a key diagnostic of the formation of giant planets1–3. The giant planets of the Solar System show an inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity4. However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b (refs. 5–9) has an atmospheric metallicity 59–276 times solar (at 1σ), which is greater than Saturn’s atmospheric metallicity of roughly 7.5 times solar10 at more than 4σ confidence. This result is based on modelling CO2 and H2O absorption features in the thermal emission spectrum of the planet measured by the James Webb Space Telescope. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66 ± 2% by mass11,12. We find that the atmospheric metallicities of both HD 149026b and the Solar System giant planets are more correlated with bulk metallicity than planet mass.

Date: 2023
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DOI: 10.1038/s41586-023-05984-y

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