JWST COMPASS: A NIRSpec G395H Transmission Spectrum of the Super-Earth GJ 357 b
Abstract We present JWST NIRSpec/G395H transmission spectroscopy observations of GJ 357 b, a warm ( T eq ≈ 525 K) super-Earth (1.2 R ⊕ , 1.84 M ⊕ ) orbiting a nearby M3-type star, with a median precision of 18 ppm and 27 ppm in NRS1 and NRS2, respectively. These precisions are obtained by binning the spectrum into 53 spectroscopic channels with a resolution of 60 pixels (around 0.02 μ m) each. Our analysis of the transmission spectrum reveals no detectable atmospheric spectral features. By comparing the observed spectrum with 1D forward models, we rule out atmospheres with mean molecular weights (MMWs) lower than 8 g mol −1 to 3 σ , as well as atmospheres with metallicities less than 300 × solar. The lack of a low-MMW primary atmosphere is consistent with a primordial H 2 -rich atmosphere having escaped, given the planet’s ≳5 Gyr age, relatively low surface gravity (log( g ) = 3.09), and its likely history of substantial incident extreme UV radiation. We conclude that GJ 357 b most likely possesses either a high-MMW secondary atmosphere, is perhaps rich in oxidized gases like CO 2 , or is a bare rock with no atmosphere. Upcoming scheduled JWST thermal emission observations could help distinguish between these scenarios by detecting signatures indicative of atmospheric heat redistribution or molecular absorption.
Authors
- Natasha E. Batalha (ORCID: https://orcid.org/0000-0003-1240-6844)
- Mercedes López‐Morales (ORCID: https://orcid.org/0000-0003-3204-8183)
- Jéa Adams Redai (ORCID: https://orcid.org/0000-0002-4489-3168)
- Nicholas Scarsdale (ORCID: https://orcid.org/0000-0003-3623-7280)
- Annabella Meech (ORCID: https://orcid.org/0000-0002-7500-7173)
- Munazza K. Alam (ORCID: https://orcid.org/0000-0003-4157-832X)
- Nicole L. Wallack (ORCID: https://orcid.org/0000-0003-0354-0187)
- Nicholas F. Wogan (ORCID: https://orcid.org/0000-0002-0413-3308)
- Johanna Teske (ORCID: https://orcid.org/0009-0008-2801-5040)
- Peter Gao (ORCID: https://orcid.org/0000-0002-8518-9601)
- Sarah E. Moran (ORCID: https://orcid.org/0000-0002-6721-3284)
- Angie Wolfgang (ORCID: https://orcid.org/0000-0003-2862-6278)
- Lili Alderson (ORCID: https://orcid.org/0000-0001-8703-7751)
- Natalie M. Batalha (ORCID: https://orcid.org/0000-0002-7030-9519)
- Hannah R. Wakeford (ORCID: https://orcid.org/0000-0003-4328-3867)
- Artyom Aguichine (ORCID: https://orcid.org/0000-0002-8949-5956)
- Anna Gagnebin (ORCID: https://orcid.org/0009-0003-2576-9422)
Institutions
- Ames Research Center (US)
- Space Telescope Science Institute (US)
- Goddard Space Flight Center (US)
- University of California, Santa Cruz (US)
- Carnegie Institution for Science (US)
- Cornell University (US)
- University of Bristol (GB)
- Université Bourgogne Franche-Comté (FR)
- Center for Astrophysics Harvard & Smithsonian (US)
- Eureka Scientific (US)
Publication Details
- Journal
- The Astronomical Journal
- Published
- 2025-09-12
- DOI
- https://doi.org/10.3847/1538-3881/adee92
- Citations
- 3
- Primary Topic
- Stellar, planetary, and galactic studies
- Type
- article
- Field-Weighted Citation Impact
- 3.46