JWST NIRISS Transmission Spectroscopy of the Super-Earth GJ 357b, a Favourable Target for Atmospheric Retention

TL;DR

This study presents the first JWST NIRISS transmission spectrum of super-Earth GJ 357b, finding no clear atmospheric features and assessing the potential for atmospheric retention and future detectability with JWST instruments.

Contribution

First atmospheric observation of GJ 357b using JWST, with analysis of atmospheric composition constraints and feasibility of future atmospheric detection.

Findings

No clear atmospheric features detected in the spectrum.

Atmospheres with metallicities below 100× solar are rejected at 3-sigma.

Detectability of atmospheric features with JWST is feasible with multiple transits.

Abstract

We present a JWST NIRISS/SOSS transmission spectrum of the super-Earth GJ 357 b: the first atmospheric observation of this exoplanet. Despite missing the first $\sim$40 % of the transit due to using an out-of-date ephemeris, we still recover a transmission spectrum that does not display any clear signs of atmospheric features. We perform a search for Gaussian-shaped absorption features within the data but find that this analysis yields comparable fits to the observations as a flat line. We compare the transmission spectrum to a grid of atmosphere models and reject, to 3-$\sigma$ confidence, atmospheres with metallicities $\lesssim$100$\times$ solar ($\sim$4 g/mol) with clouds at pressures down to 0.01 bar. We analyse how the retention of a secondary atmosphere on GJ 357 b may be possible due to its higher escape velocity compared to an Earth-sized planet and the exceptional inactivity of its host star relative to other M2.5V stars. The star's XUV luminosity decays below the threshold for rapid atmospheric escape early enough that the volcanic revival of an atmosphere of several bars of CO$_2$ is plausible, though subject to considerable uncertainty. Finally, we model the feasibility of detecting an atmosphere on GJ 357 b with MIRI/LRS, MIRI photometry, and NIRSpec/G395H. We find that, with two eclipses, it would be possible to detect features indicative of an atmosphere or surface. Further to this, with 3-4 transits, it would be possible to detect a 1 bar nitrogen-rich atmosphere with 1000 ppm of CO$_2$.