GJ 357 b: A Super-Earth Orbiting an Extremely Inactive Host Star

TL;DR

This study uses X-ray observations to estimate the age and atmospheric evolution of GJ 357 b, a Super-Earth orbiting an extremely inactive M dwarf, suggesting it likely lost its primordial atmosphere and may have a secondary atmosphere from volcanic activity.

Contribution

It provides the first detailed atmospheric evolution constraints for GJ 357 b using X-ray data and systematic stellar parameter analysis, highlighting the planet's likely atmospheric loss and potential secondary atmosphere.

Findings

GJ 357 is among the least X-ray active M dwarfs, with an age over 5 Gyr.

The initial primordial atmosphere of GJ 357 b was likely less than 38 Earth masses.

Possible secondary atmospheres include compositions rich in CO2, SO2, or N2 with trace gases.

Abstract

In this paper we present a deep X-ray observation of the nearby M dwarf GJ 357 and use it to put constraints on the atmospheric evolution of its planet, GJ 357 b. We also analyse the systematic errors in the stellar parameters of GJ 357 in order to see how they affect the perceived planetary properties. We estimate the age of GJ 357 b by comparing the observed X-ray luminosity of its host star, derived from a recent {\em XMM-Newton} observation {($\log{L_{\rm x}}\,{\rm [erg/s]} = 25.73$), with $L_{\rm x} -$ age relations for M dwarfs. We find that GJ 357 presents one of the lowest X-ray activity levels ever measured for an M dwarf, and we put a lower limit on its age of $5$\,Gyr.} Using this age limit, we perform a backwards reconstruction of the original primordial atmospheric reservoir. Furthermore, by considering the systematic errors in the stellar parameters, we find a range of possible planetary masses, radii, and densities. From the backwards reconstruction of GJ 357 b's irradiation history we find that the upper limit of its initial primordial atmospheric mass is $\sim \rm 38M_{\oplus}$. An initial atmospheric reservoir significantly larger than this may have survived through the X-ray and ultraviolet irradiation history, hence being inconsistent with current observations that suggest a telluric composition. In spite of the unlikelihood of a currently existing primordial envelope, volcanism and outgassing may have contributed to a secondary atmosphere. Under this assumption, we present three different synthetic infrared spectra for GJ 357 b that one might expect, consisting of $100\%~\rm CO_{2}$, $100\%~\rm SO_{2}$, and $75\%~ \rm N_{2}$, $24\%~\rm CO_{2}$ and $1\%~\rm H_{2}O$.