[astro-ph.EP] We present a panchromatic transmission spectrum of GJ1132b 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∼10−3bar), N2-dominated atmosphere (μ∼16amu) with traces of NH3, H2O, and CH4, combined with a grey, optically thin power-law cloud (opacity at 350~nm ≤10−3 cm2g−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 Rp=1.129±0.002R⊕, though a tenuous, obscured atmosphere cannot be excluded when the JWST first visit is included.
Similar to Fig. 1 but for the panchromatic spectra. In both panels, the spectra are displayed relative to a reference transit depth of ∼ 2418 ppm. For the Magellan/LDSS3C spectrum, a best-fit offset of ∼ +197 ppm, and for the HST/WFC3 spectrum a best-fit offset of ∼ −3 ppm are applied. Compared to the JWST-only spectrum (left panel of Fig. 1), the panchromatic transmission spectrum exhibits little evidence for the H2O feature at 5.0–5.2 m. Instead, the retrieved spectrum is primarily shaped by absorption attributed to NH3, with features near 1.2 m and across the 2.8–3.0 m wavelength range. — [astro-ph.EP]
Mohammad Senobar, Sareh Ataiee
Comments: 26 pages, accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2608.24803 [astro-ph.EP] (or arXiv:2608.24803v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2608.24803
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Submission history
From: Sareh Ataiee
[v1] Tue, 25 Aug 2026 16:47:59 UTC (20,458 KB)
https://arxiv.org/abs/2608.24803
Astrobiology
