Hubble WFC3 Spectroscopy of the Terrestrial Planets L~98-59~c \& d: No Evidence for a Clear Hydrogen Dominated Primary Atmosphere

TL;DR

This study used HST/WFC3 spectroscopy to investigate the atmospheres of two terrestrial exoplanets around L~98-59, finding no evidence for a clear hydrogen-dominated atmosphere, and highlighting the need for future JWST observations.

Contribution

First spectroscopic analysis of L~98-59~c and d's atmospheres, ruling out clear hydrogen atmospheres with HST and outlining future JWST potential.

Findings

No clear hydrogen-dominated atmospheres detected at 3 sigma confidence.

Planets may have secondary atmospheres, primary atmospheres with clouds, or no atmospheres.

Current data cannot distinguish between these atmospheric scenarios.

Abstract

The nearby bright M-dwarf star L~98-59 has three terrestrial-sized planets. One challenge remaining in characterizing atmospheres around such planets is that it is not known a priori whether they possess any atmospheres. Here we report the study of the atmospheres of L~98-59~c and L~98-59~d using the near-infrared spectral data from the G141 grism of HST/WFC3. We can reject the hypothesis of a clear atmosphere dominated by hydrogen and helium at a confidence level of $\sim$ 3~sigma for both planets. Thus they may have a primary hydrogen-dominated atmosphere with an opaque cloud layer, or have lost their primary hydrogen-dominated atmosphere and re-established a secondary thin atmosphere, or have no atmosphere at all. We cannot distinguish between these scenarios for the two planets using the current HST data. Future observations with JWST would be capable of confirming the existence of atmospheres around L~98-59~c and d and determining their compositions.