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arXiv · 2508.10579

Additional JWST/NIRSpec Transits of the Rocky M Dwarf Exoplanet GJ 1132 b Reveal a Featureless Spectrum

Katherine A. Bennett·Ryan J. MacDonald·Sarah Peacock·Junellie Perez·E. M. May·Sarah E. Moran·Lili Alderson·Jacob Lustig-Yaeger·Hannah R. Wakeford·David K. Sing·Kevin B. Stevenson·Natasha E. Batalha·Mercedes López-Morales·Munazza K. Alam·Joshua D. Lothringer·Guangwei Fu·James Kirk·Jeff A. Valenti·L. C. Mayorga·Kristin S. Sotzen
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Abstract

As an archetypal M-dwarf rocky exoplanet, GJ 1132 b has a varied history of atmospheric measurements. At 1.13 $\rm R_{\oplus}$, 1.66 $\rm M_{\oplus}$, and 580 K, it orbits a bright, slowly rotating M dwarf in a 1.6-day period, making it a prime target for characterization. In this study, we combine two JWST NIRSpec/G395H transits previously reported by May and MacDonald et al. 2023 with two new NIRSpec/G395M transits to constrain the presence of an atmosphere. This marks the first time the G395H and G395M modes have been combined for a single target, and we report no difference in the quality of data between the two modes. For rocky M-dwarf studies, G395H may still be preferred if stacking transits to utilize the high-resolution flux-calibrated stellar spectra and assess evolving stellar heterogeneity. GJ 1132 b's co-added transmission spectrum is best-fit with a flat line. A thin steam atmosphere is also consistent with the data, but this interpretation is driven almost entirely by the first transit, which suggests an increase in cool spot coverage-fraction derived from the flux-calibrated stellar spectra. This demonstrates the importance of always considering stellar heterogeneity evolution in multi-visit transits, and also the importance of a "leave-one-transit-out" approach in modeling efforts of co-added transits. We combine these results with MIRI/LRS emission data (Xue et al. 2024) to show that together, transmission and emission are consistent with only the thinnest of atmospheres. Given GJ 1132 b's age and distance from the star, a thin atmosphere is not likely stable. Therefore, the simplest explanation is that GJ 1132 b is indeed a bare rock.

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Katherine A. Bennett, Ryan J. MacDonald, Sarah Peacock, Junellie Perez, E. M. May, Sarah E. Moran, Lili Alderson, Jacob Lustig-Yaeger, Hannah R. Wakeford, David K. Sing, Kevin B. Stevenson, Natasha E. Batalha, Mercedes López-Morales, Munazza K. Alam, Joshua D. Lothringer, Guangwei Fu, James Kirk, Jeff A. Valenti, L. C. Mayorga, Kristin S. Sotzen. 2025-08-14. Additional JWST/NIRSpec Transits of the Rocky M Dwarf Exoplanet GJ 1132 b Reveal a Featureless Spectrum. https://doi.org/10.3847/1538-3881%2Fadf198

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