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

We present a panchromatic transmission spectrum of GJ1132b spanning 0.71 -- 5.16 μ 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 ref about 10-3 bar), N 2-dominated atmosphere (μ about 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 10-3 cm squared,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 p = 1.129 plus or minus 0.002,R, though a tenuous, obscured atmosphere cannot be excluded when the JWST first visit is included.