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

TL;DR

TOI-5205b, a Jupiter-like planet around an M4 dwarf, is studied with three JWST NIRSpec PRISM transits to probe atmospheric composition amid strong stellar contamination from unocculted starspots. Forward modeling (PICASO/VULCAN) and Bayesian retrievals (POSEIDON, PLATON, TauREx, etc.) converge on a metal-poor atmosphere with a high C/O ratio, with CH_4 and H_2S robustly detected while H_2O remains elusive due to stellar contamination. Interior-structure modeling (PLANETSYNTH) yields a bulk metallicity Z_planet ≈ 0.17 ± 0.07, significantly higher than the atmospheric metallicity, implying incomplete mixing or stratification between interior and atmosphere. The results highlight the dominant role of TLS in shaping the spectrum, the potential formation implications for gas giants around low-mass stars, and the need for future JWST observations (including emission spectroscopy) to corroborate the atmospheric inferences and refine formation scenarios. Overall, TOI-5205b provides a critical data point for understanding metallicity trends and interior-atmosphere decoupling in GEMS, with implications for planet formation theories around M dwarfs.

Abstract

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

Figures (43)

• Figure 1: Panels (a)-(c) JWST NIRSpec PRISM white light curves produced using ExoTiC-JEDI after binning to a cadence of 5 s. Top row: The data along with the best-fitting model (solid line) along with the residuals to the fit below. Middle row: The stellar surface and the adopted spot configuration. The solid line indicates the position of the transit chord (center of the planet) and the dashed lines mark the $\pm~R_p$ from the center of the transit chord. Bottom row: The RMS for each visit for in-transit (blue) and out-of-transit (orange) data. The prediction for Gaussian white noise is shown as a red solid line. The residuals to the model fits demonstrate there is no significant time correlated noise in-transit after modeling the spots.
• Figure 3: Top Left. Co-added TOI-5205b ExoTiC-JEDI transmission spectrum along with the best-fitting grid-based models assuming equilibrium chemistry (orange line, obtained from PICASO), equilibrium chemistry with TLS (blue dotted line), or disequilibrium chemistry with TLS (pink dotted line, obtained from VULCAN). Models with TLS cannot fully replicate the slope at the blue end of the spectrum. Bottom Left. The difference between the data and model, scaled by the errors of the data. The $\pm3\sigma$ region is shaded for reference. Right. The pressure-temperature profile for each grid shown in the top left panel.
• Figure 5: Maximum a posteriori retrieved transmission spectrum (black line) with the contributions, derived using POSEIDON, shown from stellar contamination (red line) and atmospheric opacity (blue line). The contributions from individual atmospheric species are shown with various colors (see legend). The transmission spectrum of TOI-5205b is characterized by strong stellar contamination throughout much of the $0.6 - 3.5$ µm wavelength range, with evident absorption from the planetary atmosphere due to CH4 and H2S. No other molecular species were significantly detected in our retrievals (see \ref{['tab:retrievals_clear']}).
• Figure 6: Comparison of the metallicity calibration sample from Mann2013 (black points) to our measured metallicity of TOI-5205 (red point). The metallicities are shown plotted against host star spectral type. Our measured metallicity is at the upper limit of the calibration sample.
• Figure 7: Posterior distribution of the bulk metallicity ($Z_\mathrm{planet}$) inferred with thermal evolution models. The shaded olive region denotes the atmospheric metallicity ($\log\mathrm{[M/H]}=-1.9$). The inferred bulk metallicity, much higher than the atmospheric metallicity, is incompatible with a homogeneously mixed planet.