GEMS JWST: Transmission spectroscopy of TOI-5205b reveals significant stellar contamination and a metal-poor atmosphere

• DOI: 10.48550/arxiv.2502.06966
• Publication Date: 2025-02-10
• Authors: Caleb I. Cañas; Jacob Lustig‐Yaeger; Shang‐Min Tsai; Simon Müller; Ravit Helled; Dana R. Louie; Giannina Guzmán G...; Shubham Kanodia; Peter Gao; Jessica E. Libby-...; Kevin K. Hardegre...; Knicole D. Colón; Ian Czekala; Megan Delamer; Te Sun Han; Andrea S. J. Lin; Suvrath Mahadevan; Erin May; Joe P. Ninan; Anjali A. A. Piette; Guðmundur Stefánsson; Kevin B. Stevenson; Johanna Teske; Nicole L. Wallack
• Keywords: Earth and Planetary Astrophysics (astro-ph.EP); FOS: Physical sciences; Astrophysics - Earth and Planetary Astrophysics

Recent discoveries of transiting giant exoplanets around M dwarfs (GEMS) present an opportunity to investigate their atmospheric compositions and explore how such massive planets can form low-mass stars contrary canonical formation models. Here, we the first transmission spectra TOI-5205b, a short-period ($P=1.63~\mathrm{days}$) Jupiter-like planet ($M_p=1.08~\mathrm{M_J}$ $R_p=0.94~\mathrm{R_J}$) orbiting M4 dwarf. We obtained three transits using PRISM mode JWST Near Infrared Spectrograph (NIRSpec) spanning $0.6-5.3$ um. Our data reveal significant stellar contamination that is evident in light curves as spot-crossing events larger transit depth at bluer wavelengths. Atmospheric retrievals demonstrate from unocculted star spots dominant component spectrum wavelengths $\lambda\lesssim3.0$ um, which reduced sensitivity presence clouds or hazes our The degree also prevented definitive detection any $\mathrm{H_2O}$, has primary absorption features these shorter broad wavelength coverage NIRSpec enabled robust $\mathrm{CH_4}$ $\mathrm{H_2S}$, have detectable molecular between $3.0-5.0$ gridded Bayesian consistently favored atmosphere with both sub-solar metallicity ($\log\mathrm{[M/H]}\sim-2$ for clear atmosphere) super-solar C/O ratio ($\log\mathrm{[C/O]}\sim3$ cloudy atmosphere). This contrasts estimates planetary interior models predict bulk 10--20%, $\sim100\times$ metallicity, suggests TOI-5205b decoupled its not well mixed.