Fundamental uncertainties in M-Earth transit spectra due to unconstrained climate states

Earth is the only known habitable or inhabited planet to date. However, since we are not yet able to observe the atmospheres of Earth-like planets orbiting Sun-like stars, the search for life outside our Solar system has focused on M-Earths, which are rocky planets orbiting in the habitable zones of M-dwarfs. Although JWST can, in theory, observe their atmospheres, these observations are time-consuming and difficult to interpret. M-Earth climates are also expected to differ significantly from Earth’s. Therefore, in order to make good use of telescope time, it is necessary to understand an M-Earth’s possible climate states and how these might present in observations. 

M-dwarf systems are compact, so M-Earths are expected to be tidally locked to their stars. A synchronously rotating planet must circulate heat from the substellar point to its permanent nightside in order to maintain its atmosphere. The instellation gradient gives rise to the “eyeball” climate state: a frozen nightside and a temperate region around the substellar point where liquid water can exist. An M-Earth’s habitability, in the traditional sense, depends on whether or not water is present in this region. However, the surfaces of M-Earths are not accessible to observations, so it is relevant to ask whether this information can be recovered in transit spectra.

In this work, we use the 3D climate model ExoPlaSim to simulate a vast parameter space of M-Earth climates and synthetic observations. We systematically vary dayside land cover and the mass of the atmosphere, since these variables have important climate implications, but will not be known a priori for a given planet. We find that both the amount and the location of land on the dayside determine the abundance of water vapour, which together with the atmosphere mass determines how much energy is transported to the nightside. A large range of possible climates arise from variations in these parameters. 

To determine the observational uncertainties associated with these climate differences, we generate synthetic water vapour transmission spectra from our climate simulations using petitRADTRANS. We find that the differences in water vapour abundance between simulations are recovered in the spectra, but that JWST is unlikely to be able to distinguish between different climate states from this information because the signal is too small, especially when clouds are included in the radiative transfer calculation. There is also overlap between the effects of land fraction, land configuration, and atmosphere mass on the size of the water vapour spectral feature, such that a planet’s climate state cannot be unambiguously identified from this information alone. Consequently, observers will need to account for these climate uncertainties when interpreting M-Earth spectra.