Atmospheric Retrieval of Combined JWST, HST, and Ground-Based Observations Reveals a Featureless Transmission Spectrum for GJ 1132 b

Abstract

We present a panchromatic transmission spectrum of GJ1132b spanning $0.71$--$5.16~\mu$m, combining published transit observations from Magellan/LDSS3C, HST/WFC3 G141, and JWST/NIRSpec (G395H and G395M). Using petitRADTRANS and PyMultiNest, we perform Bayesian atmospheric retrievals, comparing a flat-line, Gaussian-feature models, and clear and cloudy atmospheric scenarios, including tests with and without the first JWST visit flagged as a potential outlier in a previous study. The inclusion of shorter-wavelength spectra shifts the marginal molecular preference from H$_2$O (JWST-only) to NH$_3$ (panchromatic). Of the seven molecules tested (NH$_3$, CO$_2$, CO, CH$_4$, H$_2$O, HCN, and N$_2$O), only NH$_3$ shows a marginal preference, which is weakened when the JWST first visit is excluded. Cloudy atmospheres yield higher Bayesian evidence than high-mean-molecular-weight clear models, but none provides compelling evidence over the flat-line scenario. A thin ($P_{\rm ref} \sim 10^{-3}$~bar), N$_2$-dominated atmosphere ($\mu \sim 16$~amu) with traces of NH$_3$, H$_2$O, and CH$_4$, combined with a grey, optically thin power-law cloud (opacity at 350~nm $\le 10^{-3}~{\rm cm^2\,g^{-1}}$), remains indistinguishable from the flat-line model when the first JWST visit is included and is further disfavoured when it is excluded, strengthening a bare-rock interpretation. We conclude that current spectra show no evidence for an atmosphere on GJ1132b. The observations are best explained by a bare rocky planet with $R_{\rm p} = 1.129 \pm 0.002\,R_\oplus$, though a tenuous, obscured atmosphere cannot be excluded when the JWST first visit is included.