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Published April 2026 | Version Published
Journal Article Open

GEMS JWST: Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere

Creators

  • 1. ROR icon Goddard Space Flight Center
  • 2. NASA Postdoctoral Program Fellow
  • 3. ROR icon Johns Hopkins University
  • 4. ROR icon University of California, Riverside
  • 5. ROR icon University of Zurich
  • 6. Earth and Planets Laboratory, Carnegie Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA
  • 7. ROR icon Catholic University of America
  • 8. ROR icon University of Maryland, College Park
  • 9. ROR icon Pennsylvania State University
  • 10. ROR icon NASA Exoplanet Science Institute
  • 11. ROR icon University of St Andrews
  • 12. ROR icon University of California, Berkeley
  • 13. ROR icon California Institute of Technology
  • 14. ROR icon Tata Institute of Fundamental Research
  • 15. ROR icon University of Birmingham
  • 16. ROR icon University of Amsterdam

Abstract

Recent discoveries of transiting giant exoplanets (Rp ≳ 8 R⊕) around M dwarfs present an opportunity to investigate their atmospheric compositions and explore how such massive planets form around low-mass stars contrary to the prediction from formation models. We present the first transmission spectra of TOI-5205b, a short-period (P = 1.63 days) Jupiter-like planet (Mp = 1.08 MJ and Rp = 0.94 RJ) orbiting an M4 dwarf (M⋆ = 0.392 M⊙, R⋆ = 0.394 R⊙). We obtained three transits using the PRISM mode of the JWST Near Infrared Spectrograph spanning 0.6–5.3 μm. The data reveal significant stellar contamination that is evident in the light curves as spot-crossing events and in the transmission spectra as a larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate that stellar contamination from unocculted starspots and faculae is the dominant component of the transmission spectrum at wavelengths λ ≲ 3.0 μm, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of H2O. The wavelength coverage enabled a robust detection of CH4 and H2S, which have detectable molecular features between 3.0 and 5.0 μm. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with subsolar metallicity (3σ upper limit of log [C/O] ≳ 0.09), and supersolar C/O ratio (3σ lower limit of log [M/H] ≲ −1.24), although this may partly be driven by the nondetection of water due to stellar contamination. Planetary interior models predict a bulk metallicity of 10%–20%, which is larger than the atmospheric metallicity and suggests that the interior of TOI-5205b is decoupled from its atmosphere.

Copyright and License

© 2026. 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 the thorough and valuable feedback that has improved the quality of this manuscript. We thank (i) Adriana Kuehnel and Zafar Rustamkulov for assistance in reducing JWST data as part of the GEMS JWST survey, (ii) Michael Zhang and Ryan MacDonald for assistance with the PLATON and POSEIDON codes, respectively, and (iii) Amanda Marrione and Marcio Melendez Hernandez, members of the STScI JWST Help Desk, for assistance in confirming the nature of the tilt event in visit 2 (Obs. 17).

This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from MAST at STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program #3171. Support for program #3171 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127.

The JWST data presented in this paper were obtained from MAST at STScI. The specific observations analyzed can be accessed via doi: 10.17909/29st-dz13. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. This research has used the NASA/IPAC Infrared Science Archive, which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

C.I.C. and D.R.L. acknowledge support by NASA Headquarters through an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by ORAU through a contract with NASA, and support from NASA under award number 80GSFC24M0006. Goddard affiliates acknowledge support from the GSFC Sellers Exoplanet Environments Collaboration (SEEC), which is supported by NASA's Planetary, Astrophysics and Heliophysics Science Divisions' Research Program.

The Center for Exoplanets and Habitable Worlds is supported by the Pennsylvania State University and the Eberly College of Science.

Resources supporting this work were provided by the (i) NASA High-End Computing (HEC) Program through the NASA Center for Climate Simulation (NCCS) at Goddard Space Flight Center, (ii) Pennsylvania State University’s Institute for Computational and Data Sciences’ (ICDS) Roar supercomputer, and (iii) Carnegie Science Earth and Planets Laboratory. This content is solely the responsibility of the authors and does not necessarily represent the views of the NCCS, ICDS, or Carnegie Science.

Facilities

JWST - James Webb Space Telescope, IRSA - , IRTF - Infrared Telescope Facility.

Software References

astroquery (A. Ginsburg et al. 2019), astropy (Astropy Collaboration et al. 2013, 2018, 2022), batman (L. Kreidberg 2015), dynesty (J. S. Speagle 2020), Eureka! (T. Bell et al. 2022), ExoTiC-JEDI (L. Alderson et al. 2022), ExoTiC-LD (D. Grant & H. R. Wakeford 2024), GGChem (P. Woitke et al. 2018), jwst (H. Bushouse et al. 2023), juliet (N. Espinoza et al. 2019a), lightkurve (Lightkurve Collaboration et al. 2018), matplotlib (J. D. Hunter 2007), numpy (S. van der Walt et al. 2011), pandas (W. McKinney 2010), petitRADTRANS (P. Mollière et al. 2019), PICASO (N. E. Batalha et al. 2019; S. Mukherjee et al. 2023), planetsynth (S. Müller & R. Helled 2021), PLATON (M. Zhang et al. 2019), POSEIDON (R. J. MacDonald & N. Madhusudhan 2017), pyMSG (R. Townsend & A. Lopez 2023), PyMultiNest (J. Buchner et al. 2014), scipy (P. Virtanen et al. 2020), spotrod (B. Béky et al. 2014), Spextool (M. C. Cushing et al. 2004), TauREx (A. F. Al-Refaie et al. 2021, 2022), VULCAN (S.-M. Tsai et al. 2017, 2021).

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2502.06966 (arXiv)
Is supplemented by
Dataset: 10.17909/29st-dz13 (DOI)

Funding

Space Telescope Science Institute
JWST-GO-3171
National Aeronautics and Space Administration
NAS 5-03127
National Aeronautics and Space Administration
80GSFC24M0006

Dates

Submitted
2025-02-10
Accepted
2026-01-27
Available
2026-03-31
Published

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Caltech groups
Infrared Processing and Analysis Center (IPAC) , Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published