Additional JWST/NIRSpec transits of the rocky M dwarf exoplanet GJ 1132 b reveal a featureless spectrum

As an archetypal M-dwarf rocky exoplanet, GJ 1132 b has a varied history of atmospheric measurements. At 1.13 R_⊕, 1.66 M_⊕, 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 E. M. May 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 coadded 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 multivisit transits, and also the importance of a “leave-one-transit-out” approach in modeling efforts of coadded transits. We combine these results with MIRI/LRS emission data 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.