Characterization of Rocky Exoplanets in Thermal Emission with JWST

The spectroscopic characterization of terrestrial exoplanets will be made possible for the first time with JWST. A key first-order question that we wish to address is whether such planets possess atmospheres at all. If they do, we will endeavor to measure their atmospheric thickness, composition, and cloud properties. If instead terrestrial planets’ atmospheres have been lost to space, the opportunity emerges to characterize their solid surfaces. This talk will focus on the information that can be gleaned from thermal emission measurements for rocky exoplanets. I will briefly describe how the presence and thickness of an atmosphere can be established via emission photometry and how, for terrestrial exoplanets interior the habitable zone, this is the most efficient and unambiguous way to identify the existence of an atmosphere (Koll et al., 2019; Mansfield et al., 2019). I will then present a new study of the planetary emission of the benchmark terrestrial planet LHS 3844b, including both atmospheric and surface contributions to the overall thermal emission (Whittaker et al., in prep). From our models, we find that most plausible atmospheres of 1 bar or less in thickness are ruled out based on existing Spitzer data from Kreidberg et al., 2019. We furthermore find that upcoming JWST data have the potential to further constrain the atmospheric and surface composition of LHS 3844b, but care must be taken to account for the combined effects of both atmosphere and surface on the thermal emission spectrum. Traditional retrievals that do not account for the planetary surface can result in biased measurements of atmospheric abundances or even falsely detect the signatures of absent species. I will conclude by summarizing the upcoming set of rocky exoplanets that will be characterized in thermal emission with JWST during Cycle 1 and offer some thoughts on what scientific insights can be drawn from these pioneering data sets.