The Atmospheric Circulation and Climate of Terrestrial Planets Orbiting Sun-like and M Dwarf Stars over a Broad Range of Planetary Parameters

Abstract

The recent detections of temperate terrestrial planets orbiting nearby stars and the promise of characterizing their atmospheres motivates a need to understand how the diversity of possible planetary parameters affects the climate of terrestrial planets. In this work, we investigate the atmospheric circulation and climate of terrestrial exoplanets orbiting both Sun-like and M-dwarf stars over a wide swath of possible planetary parameters, including the planetary rotation period, surface pressure, incident stellar flux, surface gravity, planetary radius, and cloud particle size. We do so using a general circulation model (GCM) that includes non-grey radiative transfer and the effects of clouds. The results from this suite of simulations generally show qualitatively similar dependencies of circulation and climate on planetary parameters as idealized GCMs, with quantitative differences due to the inclusion of additional model physics. Notably, we find that the effective cloud particle size is a key unknown parameter that can greatly affect the climate of terrestrial exoplanets. We confirm a transition between low and high dayside cloud coverage of synchronously rotating terrestrial planets with increasing rotation period. We determine that this cloud transition is due to eddy-driven convergence near the substellar point and should not be parameterization-dependent. Finally, we compute full-phase light curves from our simulations of planets orbiting M-dwarf stars, finding that changing incident stellar flux and rotation period affect observable properties of terrestrial exoplanets. Our GCM results can guide expectations for planetary climate over the broad range of possible terrestrial exoplanets that will be observed with future space telescopes.

Habitability and Spectroscopic Observability of Warm M-dwarf Exoplanets Evaluated with a 3D Chemistry-Climate Model

“…In our simulations, reduced OLR coincides with locations of deep convective clouds as high clouds exhibit a strong warming effect, and substantially reduce OLR (Figure 3i (Matsuno 1966;Gill 1980). Indeed, we find that lower rotation period planet atmospheres around later M-dwarfs (lower T eff ) lead to increased turbulent flows, formation of midlatitude and super-rotating jet streams, and greater day-to-nightside asymmetry (Figure 3i-j; see also Komacek & Abbot 2019). Importantly, these dynamical transitions are largely responsible for the changes in cloud formation, a key factor in regulating the efficiency of planetary albedo, surface temperature, and thermal equilibrium.…”

Examining the role of varying surface pressure in the climate of early Earth

“…Therefore, the meridional temperature gradient decreases with increasing air pressure, and the tropical trade winds and extratropical eddies and jets become weaker in strength and smaller in length scales. The reduced horizontal surface temperature gradient with air pressure was also confirmed in the simulations with other global models (Komacek and Abbot, 2019) and even on tidally locked exoplanets (such as Yang et al (2019)). (4) Background air pressure also influences the thermal stratification of the atmosphere through its effect on the moist adiabatic lapse rate, which is given by g R sd T 2 +LvrT c pd R sd T 2 +L 2 v r , where g is gravitational acceleration, L v is the heat of water vapor condensation, R sd is the specific gas constant of dry air, r is the ratio of the mass of water vapor to the mass of dry air, and is the ratio of the specific gas constant for dry air to the specific gas constant for water vapor (Stone and Carlson, 1979;Charnay et al, 2013).…”

Aerosols in Exoplanet Atmospheres

“…Yang et al, 2013). Such cloud patterns are predicted to be sensitive to planet rotation rate (Komacek & Abbot, 2019;J. Yang et al, 2014) and lead to significant muting of molecular features in transmission Suissa et al, 2020).…”