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

TL;DR

This study combines multiple JWST transits to analyze GJ 1132 b's atmosphere, finding a featureless spectrum consistent with a bare rocky surface and emphasizing the importance of accounting for stellar heterogeneity.

Contribution

First combined G395H and G395M JWST transit data for GJ 1132 b, revealing a flat transmission spectrum and highlighting stellar heterogeneity effects in atmospheric characterization.

Findings

GJ 1132 b's spectrum is featureless, indicating no detectable atmosphere.

A thin steam atmosphere cannot be ruled out but is unlikely stable given the planet's age.

Stellar heterogeneity significantly impacts multi-visit transit analyses.

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.