2022MNRAS.511.2565P

Emission spectroscopy is a promising technique to observe atmospheres of rocky exoplanets, probing both their chemistry and thermal profiles. We present HYDRo, an atmospheric retrieval framework for thermal emission spectra of rocky exoplanets. HYDRo does not make prior assumptions about the background atmospheric composition, and can therefore be used to interpret spectra of secondary atmospheres with unknown compositions. We use HYDRo to assess the chemical constraints which can be placed on rocky exoplanet atmospheres using JWST. First, we identify the best currently known rocky exoplanet candidates for spectroscopic observations in thermal emission with JWST, finding >30 known rocky exoplanets whose thermal emission will be detectable by JWST/MIRI in fewer than 10 eclipses at R ∼ 10. We then consider the observations required to characterize the atmospheres of three promising rocky exoplanets across the ∼400-800 K equilibrium temperature range: Trappist-1 b, GJ 1132 b, and LHS 3844 b. Considering a range of CO₂- to H₂O-rich atmospheric compositions, we find that as few as eight eclipses of LHS 3844 b or GJ 1132 b with MIRI LRS will be able to place important constraints on the chemical compositions of their atmospheres. This includes confident detections of CO₂ and H₂O in the case of a cloud-free CO₂-rich composition, besides ruling out a bare rock scenario. Similarly, 30 eclipses of Trappist-1 b with MIRI LRS can allow detections of a cloud-free CO₂-rich or CO₂-H₂O atmosphere. HYDRo will allow important atmospheric constraints for rocky exoplanets using JWST observations, providing clues about their geochemical environments.