Understanding the climates and habitability of Earth-like exoplanets is important for assessing the potential for life beyond our Solar System. These planets are influenced by a complex interplay of factors, including host star type, atmospheric composition, surface characteristics, and orbital configurations such as high eccentricity and tidal locking. Many rocky exoplanets exhibit higher orbital eccentricities than those in our Solar System, and terrestrial exoplanets orbiting M dwarfs are often tidally-locked due to their proximity to the host stars.
In this thesis, I employ the Whole Atmosphere Community Climate Model Version 6 (WACCM6), a fully coupled Earth-system model, to simulate and examine the climates of Earth-like exoplanets in highly eccentric and tidally-locked orbits. I aim to provide a basic understanding of how these two orbital parameters influence planetary climate, surface habitability and observational signatures. My simulations reveal that orbital eccentricity and tidal locking both have a significant and distinct impact on the climate. A highly eccentric Earth-like exoplanet (e = 0.4) exhibits a warmer global mean surface temperature than its circular orbit counterpart (e = 0) due to lower surface and cloud albedo and weaker longwave cloud forcing. The seasonally asymmetric stellar irradiance in the highly eccentric orbit redistributes surface albedo feedback and cloud radiative effects spatially and temporally. This leads to an increased surface temperature, with enhanced meridional heat transport when the planet is closer to its star, improving habitability at higher latitudes. Additionally, we find a non-linear rise in stratospheric water vapor concentration as surface temperatures increase, which drives a corresponding annual mean increase in water loss rates via the thermal escape of hydrogen species. Despite the higher water loss rate, the highly eccentric Earth-like planet can retain its water reservoir throughout the system's lifetime. For a case study of the tidally-locked Proxima Centauri b with the substellar point over the Pacific Ocean, our results reveal a colder climate than pre-industrial Earth, attributed to lower stellar irradiance, higher surface albedo, and reduced greenhouse effects. Nonetheless, the planet sustains an "eyeball-shaped" open ocean around the substellar point. The stratospheric ozone layer is significantly thinner than that of the pre-industrial Earth, largely due to reduced atomic oxygen production resulting from the lower photolysis rate of molecular oxygen and water vapor, a consequence of the host star's weaker ultraviolet radiation. Notably, atmospheric circulation, ocean dynamics and topography are critical in modulating the day-night temperature contrast and shaping the ozone distribution of the tidally-locked planet. We find that the surface ozone levels remain below harmful concentrations, further supporting the potential habitability in the open ocean region.
Using the Planetary Spectrum Generator, we simulate idealized spectra for both studies considering eccentricity and tidal locking effects. The highly eccentric planet shows stronger water vapor absorption features, suggesting that such exoplanets may be prime targets for future atmospheric characterization. For the tidally-locked planet, water vapor signals are much weaker despite the planet being an ocean world. Nevertheless, ozone and carbon dioxide produce the more notable absorption features in the spectra, indicating a higher possibility of detection through direct imaging and transmission spectroscopy.
Overall, this thesis highlights the profound impact of orbital configurations, such as eccentricity and tidal locking, on the climate dynamics, atmospheric composition, potential habitability and theoretical observations of Earth-like exoplanets. My research addresses the necessity of using comprehensive three-dimensional global circulation models coupled with interactive chemistry and ocean dynamics to better assess the spatial and temporal variations in climate. This research contributes to the broader quest to identify and characterize potentially habitable worlds beyond our Solar System.
