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

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 μ m. 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 μ m, reducing the sensitivity to the presence of clouds or hazes in our models and preventing detection of H 2 O. The wavelength coverage enabled a robust detection of CH 4 and H 2 S, which have detectable molecular features between 3.0 and 5.0 μ m. For both clear or cloudy atmospheres, Bayesian retrievals consistently favored an atmosphere with subsolar metallicity (3 σ upper limit of log [ M / H ] ≲ − 1.24 ) and supersolar C/O ratio (3 σ 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.