Assessing the prevalence of atmospheres on rocky planets around M-dwarf stars is a top priority of exoplanet science. High-energy activity from M dwarfs can destroy the atmospheres of these planets, which could explain the lack of atmosphere detections to date. Volcanic outgassing has been proposed as a mechanism to replenish the atmospheres of tidally heated rocky planets. L 98-59 b, a sub-Earth transiting a nearby M dwarf, was recently identified as the most promising exoplanet to detect a volcanic atmosphere. We present the transmission spectrum of L 98-59 b from four transits observed with JWST NIRSpec G395H. Although the airless model provides an adequate fit to the data based on its χ2, an SO2 atmosphere is preferred by 3.6σ over a flat line in terms of the Bayesian evidence. Such an atmosphere would likely be in a steady state where volcanism balances escape. If so, L 98-59 b must experience at least eight times as much volcanism and tidal heating per unit mass as Io. If volcanism is driven by runaway melting of the mantle, we predict the existence of a subsurface magma ocean in L 98-59 b extending up to Rp ∼ 60%–90%. An SO2-rich volcanic atmosphere on L 98-59 b would be indicative of an oxidized mantle with an oxygen fugacity of fO2 > IW + 2.7, and it would imply that L 98-59 b must have retained some of its volatile endowment despite its proximity to its star. Our findings suggest that volcanism may revive secondary atmospheres on tidally heated rocky planets around M dwarfs.
Evidence for a Volcanic Atmosphere on the Sub-Earth L 98-59 b
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Jet Propulsion Lab
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Johns Hopkins University
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California Institute of Technology
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Arizona State University
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University of Michigan–Ann Arbor
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Michigan State University
Abstract
Copyright and License
© 2025. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
We thank the anonymous referee for providing a thoughtful and insightful report on this work. This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-03127. These observations are associated with program #3942. Support for program #3942 was provided by NASA through a grant from the Space Telescope Science Institute under NASA contract NAS5-03127. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). Part of the high-performance computing resources used in this investigation were provided by funding from the JPL Information and Technology Solutions Directorate. R.J.M. is supported by NASA through the NASA Hubble Fellowship grant HST-HF2-51513.001, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. D.Z.S. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2303553. This research award is partially funded by a generous gift of Charles Simonyi to the NSF Division of Astronomical Sciences. The award is made in recognition of significant contributions to Rubin Observatory's Legacy Survey of Space and Time.
Data Availability
The JWST NIRSpec data used in this work will be publicly available at the end of the one-year exclusive access period in the Mikulski Archive for Space Telescopes (MAST): doi:10.17909/3g7x-b466. The transmission spectra presented in this work are available on Zenodo: doi:10.5281/zenodo.14676142.
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Additional details
- National Aeronautics and Space Administration
- NAS5-03127
- Space Telescope Science Institute
- National Aeronautics and Space Administration
- 80NM0018D0004
- Jet Propulsion Laboratory
- National Aeronautics and Space Administration
- HST-HF2-51513.001
- National Aeronautics and Space Administration
- NAS 5-26555
- National Science Foundation
- AST-2303553
- Heising-Simons Foundation
- 51 Pegasi b Fellowship -
- National Aeronautics and Space Administration
- NASA Hubble Fellowship Program Sagan Fellowship -
- National Science Foundation
- NSF Astronomy and Astrophysics Postdoctoral Fellowship -
- Accepted
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2025-01-23Accepted
- Available
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2025-02-13Published
- Caltech groups
- Division of Geological and Planetary Sciences
- Publication Status
- Published