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 days) Jupiter-like planet
(
Mp=1.08 MJ and
Rp=0.94 RJ) 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≲3.0 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
H2O, which has
primary absorption features at these shorter wavelengths. The broad wavelength
coverage of NIRSpec PRISM enabled a robust detection of
CH4 and
H2S, 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[M/H]∼−2 for a clear atmosphere)
and super-solar C/O ratio (
log[C/O]∼3 for a clear or cloudy
atmosphere). This contrasts with estimates from planetary interior models that
predict a bulk metallicity of 10--20%, which is
∼100× the atmospheric
metallicity, and suggests that the planetary interior for TOI-5205b is
decoupled from its atmosphere and not well mixed.