TY - JOUR
T1 - GEMS JWST
T2 - Transmission Spectroscopy of TOI-5205b Reveals Significant Stellar Contamination and a Metal-poor Atmosphere
AU - Cañas, Caleb I.
AU - Lustig-Yaeger, Jacob
AU - Tsai, Shang Min
AU - Müller, Simon
AU - Helled, Ravit
AU - Kanodia, Shubham
AU - Louie, Dana R.
AU - Guzmán Caloca, Giannina
AU - Gao, Peter
AU - Libby-Roberts, Jessica
AU - Hardegree-Ullman, Kevin K.
AU - Colón, Knicole D.
AU - Czekala, Ian
AU - Delamer, Megan
AU - Han, Te
AU - Lin, Andrea S.J.
AU - Mahadevan, Suvrath
AU - May, Erin M.
AU - Ninan, Joe P.
AU - Piette, Anjali A.A.
AU - Stefánsson, Guðmundur
AU - Stevenson, Kevin B.
AU - Teske, Johanna
AU - Wallack, Nicole L.
N1 - Publisher Copyright:
© 2026. The Author(s). Published by the American Astronomical Society.
PY - 2026/4/1
Y1 - 2026/4/1
N2 - 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 [ 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.
AB - 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 [ 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.
UR - https://www.scopus.com/pages/publications/105035355031
UR - https://www.scopus.com/pages/publications/105035355031#tab=citedBy
U2 - 10.3847/1538-3881/ae4976
DO - 10.3847/1538-3881/ae4976
M3 - Article
AN - SCOPUS:105035355031
SN - 0004-6256
VL - 171
JO - Astronomical Journal
JF - Astronomical Journal
IS - 4
M1 - 260
ER -