A combined analysis of spectra from Magellan, HST and JWST, spanning 14 transits of GJ 1132 b, found no convincing evidence for a substantial atmosphere. The resulting transmission spectrum remained largely featureless.
The authors conclude that GJ 1132 b is most likely a bare rocky world, while not fully excluding a tenuous atmosphere. They state that the atmospheric interpretation from the combined transmission data alone cannot be considered robust in isolation.
The flat-line model ranked highest
The researchers compared four descriptions of the transmission data: a flat line with no atmospheric features, a Gaussian-shaped feature, and clear and cloudy atmospheric scenarios. They repeated the comparison with and without the first JWST visit. Of the seven molecules tested, only NH3 showed a marginal preference, and that preference weakened when the first visit was excluded.
The flat-line model had the highest Bayesian evidence among the tested models, a score used here to compare the alternatives. Its retrieved planetary radius was 1.129 ± 0.002 times Earth's radius.
The reported log Bayes factors were +1.42 for the cloudy model and +0.14 for the clear model when all visits were included. After the first visit was excluded, they were +1.05 and −0.50, respectively.
In the all-visit omission test, the version without NH3 had the largest reported loss of Bayesian evidence, with Δ ln Z = −1.13. The NH3 finding was described as a marginal preference rather than a confirmed molecular detection, and the weak evidence disappeared when the first JWST visit was excluded.
A thin atmosphere remains possible
One tested scenario was a thin, N2-dominated atmosphere with a reference pressure near 10−3 bar and a mean molecular weight near 16 atomic mass units. It included trace NH3, water and methane, along with a gray, optically thin power-law cloud. With the first JWST visit included, this scenario remained indistinguishable from the flat-line model; it was more strongly disfavored when the visit was excluded.
These are constraints on a model scenario, not a confirmed atmospheric composition. The analysis cannot cleanly separate a planet's surface pressure from the pressure at the top of a gray cloud deck, and several molecular results are dominated by the assumptions in their prior ranges.
Emission data leave the choice open
A separate comparison with the emission data disfavored abundant absorbers and optically thick clouds. It remained consistent with either bare rock or a tenuous trace-gas atmosphere, with or without a thin flat cloud.
The Bayesian retrievals used petitRADTRANS and PyMultiNest nested sampling, with 2,000 live points.
Adding a jitter term did not increase the evidence for either the flat-line or cloudy scenario.
Taken together, the analysis found no convincing evidence for a substantial atmosphere, while the tested data still permit a tenuous atmosphere in some configurations. The authors describe GJ 1132 b as most likely a bare rocky world, with the thinner alternative not fully excluded.
The document is a preprint dated 26 August 2026. The analyzed transmission spectra are stated to be publicly available from the NASA Exoplanet Archive.
Paper data and sources
Original title: Atmospheric Retrieval of Combined JWST, HST, and Ground-Based Observations Reveals a Featureless Transmission Spectrum for GJ 1132 b
Authors: Mohammad Senobar, Sareh Ataiee
Journal/Repository: arXiv
Status: Preprint, not yet peer-reviewed
First online: 2026-08-25
DOI: Not available
Original paper · Full text