JWST COMPASS: A NIRSpec G395H Transmission Spectrum of the Super-Earth GJ 357 b

TL;DR

This study uses JWST NIRSpec transmission spectroscopy to analyze GJ 357 b, finding no detectable atmospheric features and constraining its atmospheric composition, suggesting it likely has a high-MMW atmosphere or no atmosphere at all.

Contribution

First JWST NIRSpec transmission spectrum of GJ 357 b, providing constraints on its atmospheric composition and ruling out low-MMW atmospheres.

Findings

No detectable atmospheric spectral features.

Ruling out atmospheres with MMW less than 8 g/mol.

Likely a high-MMW secondary atmosphere or no atmosphere.

Abstract

We present JWST NIRSpec/G395H transmission spectroscopy observations of GJ 357 b, a warm ($T_{\mathrm{eq}} \approx 525$ K) super-Earth ($1.2\ \mathrm{R_{\oplus}} $, $1.84\ \mathrm{M_{\oplus}} $) 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 $\mu$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 (MMW) lower than 8 g/mol to $3 \sigma$, as well as atmospheres with metallicities less than 300x solar. The lack of a low MMW primary atmosphere is consistent with a primordial H$_2$ rich atmosphere having escaped, given the planet's $\gtrsim5$ Gyr age, relatively low surface gravity (log g = 3.09), and its likely history of substantial incident extreme ultraviolet radiation. We conclude that GJ 357 b most likely possesses either a high-MMW secondary atmosphere, 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.