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 2O . The wavelength coverage enabled a robust detection of CH 4 and H 2S , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">M</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">H</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo>≲</mml:mo> <mml:mo>−</mml:mo> <mml:mn>1.24</mml:mn> </mml:math> ) and supersolar C/O ratio (3 σ lower limit of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">[</mml:mo> <mml:mi mathvariant="normal">C</mml:mi> <mml:mo>/</mml:mo> <mml:mi mathvariant="normal">O</mml:mi> <mml:mo stretchy="false">]</mml:mo> <mml:mo>≳</mml:mo> <mml:mn>0.09</mml:mn> </mml:math> ), 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.