Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H

TL;DR

This study presents two JWST NIRSpec G395H transit observations of GJ 1132 b, revealing significant differences between visits and emphasizing the need for multiple observations to confirm planetary atmospheres.

Contribution

First multi-visit JWST observations of GJ 1132 b demonstrate atmospheric variability or noise, highlighting the importance of repeatability in exoplanet atmospheric characterization.

Findings

One transit suggests a possible H2O-dominated atmosphere with methane and N2O.

The other transit appears featureless, indicating variability or noise.

Results emphasize the necessity of multiple observations for reliable atmospheric detection.

Abstract

The search for rocky planet atmospheres with JWST has focused on planets transiting M dwarfs. Such planets have favorable planet-to-star size ratios, enhancing the amplitude of atmospheric features. Since the expected signal strength of atmospheric features is similar to the single-transit performance of JWST, multiple observations are required to confirm any detection. Here, we present two transit observations of the rocky planet GJ 1132 b with JWST NIRSpec G395H, covering 2.8-5.2 $\mu$m. Previous HST WFC3 observations of GJ 1132 b were inconclusive, with evidence reported for either an atmosphere or a featureless spectrum based on analyses of the same dataset. Our JWST data exhibit substantial differences between the two visits. One transit is consistent with either a H$_2$O-dominated atmosphere containing ~1% CH$_4$ and trace N$_2$O ($\chi^{2}_{\nu}$ = 1.13) or stellar contamination from unocculted starspots ($\chi^{2}_{\nu}$ = 1.36). However, the second transit is consistent with a featureless spectrum. Neither visit is consistent with a previous report of HCN. Atmospheric variability is unlikely to explain the scale of the observed differences between the visits. Similarly, our out-of-transit stellar spectra show no evidence of changing stellar inhomogeneity between the two visits - observed 8 days apart, only 6.5% of the stellar rotation rate. We further find no evidence of differing instrumental systematic effects between visits. The most plausible explanation is an unlucky random noise draw leading to two significantly discrepant transmission spectra. Our results highlight the importance of multi-visit repeatability with JWST prior to claiming atmospheric detections for these small, enigmatic planets.