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

Abstract

Abstract We present a panchromatic transmission spectrum of GJ 1132 b spanning 0.71–5.16 μm, 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 H2O (JWST-only) to NH3 (panchromatic). Of the seven molecules tested (NH3, CO2, CO, CH4, H2O, HCN, and N2O), only NH3 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 (Pref ∼ 10−3 bar), N2-dominated atmosphere (μ ∼ 16 amu) with traces of NH3, H2O, and CH4, combined with a grey, optically thin power-law cloud (opacity at 350 nm ≤10−3 cm2 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 GJ 1132 b. The observations are best explained by a bare rocky planet with Rp = 1.129 ± 0.002 R⊕, though a tenuous, obscured atmosphere cannot be excluded when the JWST first visit is included.