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

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 (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.