The search for atmospheres on rocky exoplanets is one of the major science goals defining the early JWST era. Planets which orbit M dwarfs are the most amenable to characterisation using JWST, yet we now have evidence that a number of these planets likely lack substantial atmospheres, particularly for planets close to their host stars (e.g. Greene et al. 2023; Zieba et al. 2023; Mansfield et al. 2024). The future success of these searches will have important implications for which planets we can expect to have retained atmospheres (e.g. the cosmic shoreline model; Zahnle & Catling 2017) and whether the future of terrestrial exoplanet atmosphere characterisation will be largely restricted to further separated planets observed via direct-imaging/nulling interferometry or if we will also have multiple close-in transiting exoplanets to compare these to.
I will present recent work from the Hot Rocks Survey (Fortune et al., submitted) which uses 15 micron eclipse photometry of LHS 1140c with MIRI/Imaging to determine whether it hosts an atmosphere. By comparing the level of thermal emission to various atmospheric models, we were able to rule out a wide range of atmospheres including 10 mbar pure CO2 atmospheres and 1 bar pure H2O atmospheres to greater than 3 sigma. We instead found our eclipse depth was most consistent with a low albedo bare rock finding a brightness temperature of 561±44 K, close to the theoretical maximum. This has strong implications for the cosmic shoreline model of atmospheric escape as LHS 1140c is one of the lowest instellation M dwarf targets studied so far which appears to lack a substantial atmosphere. This suggests we need to dedicate time pushing towards targets further from their host stars to find rocky planets which have retained their atmospheres.
We also identified a pattern in the initial ~30-60 minutes of detector settling which appears strongly connected to the previous filter used by MIRI. This helps explain the significant differences in initial detector settling seen across datasets in the Hot Rocks Survey and is an important effect to understand given the 500 hour Rocky Worlds DDT program extending this mid-infrared photometry technique to more targets. We also developed a new method to analyse these data which joint-fits individual pixel light curves and which sheds a new light on the systematics and persistence effects seen in MIRI/Imaging data. These effects may be important to take into consideration when designing future mid-infrared facilities such as LIFE.
