Exoplanets & Exomoons

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

By Keith Cowing
Status Report
October 19, 2023
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Double Trouble: Two Transits of the Super-Earth GJ 1132 b Observed with JWST NIRSpec G395H
Transmission spectra of GJ 1132 b from all three data reduction pipelines and both visits. Upper panel: the Eureka! (yellow), FIREFLy (pink), and ExoTiC-JEDI (blue) reductions are shown for both visits at R∼100. Also shown are the differences between reductions (solid, dashed, and dotted gray lines) in units of σ, demonstrating the agreement between pipelines within a single visit. A ±1σ shaded region is overlaid. Bottom panel: the Eureka! reduction for both visits, with the NIRSpec detector gap denoted by the shaded gray region. The shaded yellow regions show two important wavelength ranges that differ the most between the visits (see Sections 4.1 and 4.4.1 for further discussion on these differences). — astro-ph.EP

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 μm. 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 H2O-dominated atmosphere containing ~1% CH4 and trace N2O (χ2ν = 1.13) or stellar contamination from unocculted starspots (χ2ν = 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.

E. M. May, Ryan J. MacDonald, Katherine A. Bennett, Sarah E. Moran, Hannah R. Wakeford, Sarah Peacock, Jacob Lustig-Yaeger, Alicia N. Highland, Kevin B. Stevenson, David K. Sing, L. C. Mayorga, Natasha E. Batalha, James Kirk, Mercedes Lopez-Morales, Jeff A. Valenti, Munazza K. Alam, Lili Alderson, Guangwei Fu, Junellie Gonzalez-Quiles, Joshua D. Lothringer, Zafar Rustamkulov, Kristin S. Sotzen

Comments: 22 pages, 10 figures, 2 tables. Accepted for publication in ApJ Letters. Co-First Authors. Bonus materials and spectral data: this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2310.10711 [astro-ph.EP] (or arXiv:2310.10711v1 [astro-ph.EP] for this version)
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Submission history
From: Ryan MacDonald
[v1] Mon, 16 Oct 2023 18:00:00 UTC (5,255 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻