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

April 2026 • 2026AJ....171..260C

Authors • Cañas, Caleb I. • Lustig-Yaeger, Jacob • Tsai, Shang-Min • Müller, Simon • Helled, Ravit • Kanodia, Shubham • Louie, Dana R. • Guzmán Caloca, Giannina • Gao, Peter • Libby-Roberts, Jessica • Hardegree-Ullman, Kevin K. • Colón, Knicole D. • Czekala, Ian • Delamer, Megan • Han, Te • Lin, Andrea S. J. • Mahadevan, Suvrath • May, Erin M. • Ninan, Joe P. • Piette, Anjali A. A. • Stefánsson, Guðmundur • Stevenson, Kevin B. • Teske, Johanna • Wallack, Nicole L.

Abstract • Recent discoveries of transiting giant exoplanets (R_p ≳ 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 (M_p = 1.08 M_J and R_p = 0.94 R_J) 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[M/H]≲−1.24 ) and supersolar C/O ratio (3σ lower limit of log[C/O]≳0.09 ), 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.

Links

• SIMBAD https://simbad.u-strasbg.fr/simbad/sim-ref?querymethod=bib&simbo=on&submit=submit+bibcode&bibcode=2026AJ....171..260C
• PDF https://iopscience.iop.org/article/10.3847/1538-3881/ae4976/pdf
• PREPRINT http://arxiv.org/abs/2502.06966
• DATA https://archive.stsci.edu/mastbibref.php?bibcode=2026AJ....171..260C
• DATA https://doi.org/10.17909/29st-dz13
• DATA https://doi.org/10.5281/zenodo.7542068
• DATA https://doi.org/10.5281/zenodo.7874921
• ELECTR https://doi.org/10.3847/1538-3881/ae4976