2025AJ....170..219A

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 μm) 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 3σ, as well as atmospheres with metallicities less than 300 × solar. The lack of a low-MMW primary atmosphere is consistent with a primordial H₂-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 CO₂, 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.