Terminator region of tidally locked M-dwarf exoplanets in 3-D general circulation models

Abstract

The impressive sensitivity of the James Webb Space Telescope has made it possible to study the atmospheres of planets beyond the solar system. It will soon be followed by space missions aiming specifically at this goal, such as the Ariel mission, Twinkle, and the Habitable Worlds Observatory. One category of exoplanet has drawn interest because of its potential to harbour temperate climates with liquid surface water—and therefore potentially life. These are rocky planets orbiting cool M-class stars, or "M-Earths." Stellar population trends and observing biases lead to a high proportion of potentially habitable, terrestrial planets falling into this category. Because of the low temperatures of their host stars, however, habitable worlds of this type are found in close orbits where they are likely to be tidally locked. As the solar system has no tidally locked planets, our knowledge of their atmospheric circulation is currently limited to theoretical modelling.

Past modelling work has shown that the asymmetrical irradiation of tidally locked planets results in characteristic circulation regimes which have profound consequences for observations. Atmospheric retrievals, which use statistical methods to fit 1-D atmospheric models to observational data and quantify the confidence of the fit, are not yet able to account for the 3-D nature of this circulation. For planets with large spatial variation in environmental conditions caused by tidal locking, 1-D models are not able to capture the differences and interconnections between planetary regions such as the dayside, nightside, and planetary limb or terminator. In addition, planetary atmospheres exhibit variation over time, potentially resulting in differences in retrieved properties between observing visits or even between different phases of a planet’s orbit. Accounting for 4-D circulation effects in atmospheric retrievals first requires a theoretical understanding of the impact of global-scale phenomena such as atmospheric waves and horizontal transport on conditions at the planetary limb, and then requires incorporation of this knowledge into the retrieval pipeline in the form of, for example, parameterisations.

In this thesis, I address the first requirement: the theoretical understanding of the effects of fully modelled 4-D atmospheric circulation on the planetary limb, the region probed by transmission spectroscopy, on tidally locked planets. I focus in particular on effects caused by the global propagation of atmospheric waves and by horizontal transport of clouds and hazes. In Chapter 2, I show that that the atmospheric dynamics on the tidally locked Proxima Centauri b support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth’s quasi-biennial oscillation (QBO). The LASO has a vertical extent of 35–55 km, a period of 5–6.5 months, and a peak-to-peak wind speed amplitude of -70 to +130 ms−1 with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localised in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapour and trace gases such as ozone which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundance of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets.

In Chapter 3, I investigate the presence of cloud cover at the planetary limb of water-rich Earth-like planets, which is likely to weaken chemical signatures in transmission spectra and impede attempts to characterise these atmospheres. Based on observations of Earth and solar system worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. I identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber-1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres and results in advection of clouds to or away from the planet’s eastern terminator. I study this oscillation in simulations of Proxima Centauri b, TRAPPIST 1-e, and rapidly rotating versions of these worlds located at the inner edge of their stars’ habitable zones. I simulate time series of the transit depths of the 1.4 µm water feature and 2.7 µm carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of my simulations have variability significant enough to be detectable with current methods. In Chapter 4, I study the interaction between the atmospheric circulation and photochemical hazes and describe the resulting haze abundances at the terminator. Transmission spectroscopy supports the presence of unknown, light-scattering aerosols in the atmospheres of many exoplanets. The complexity of factors influencing the formation, 3-D transport, radiative impact, and removal of aerosols makes it challenging to match theoretical models to the existing data. My study simplifies these factors to focus on the interaction between planetary general circulation and haze distribution at the planetary limb. I use an intermediate complexity general circulation model, ExoPlaSim, to simulate idealised organic haze particles as radiatively active tracers in the atmospheres of tidally locked terrestrial planets for a range of rotation rates. I find three distinct 3-D spatial haze distributions, corresponding to three circulation regimes, each with a different haze profile at the limb. All regimes display significant terminator asymmetry. In my parameter space, super-Earth-sized planets with rotation periods greater than 13 days have the lowest haze optical depths at the terminator, supporting the choice of slower rotators as observing targets.

My thesis supports the existence of characteristic forms of temporal and spatial variability on tidally locked planets which will undoubtedly impact observations and inform our understanding of climate conditions on the surface. Overall, the effects of purely dynamical variability may be too small to be detected for Earth-like planets (but potentially detectable for larger ones). The impact of the atmospheric circulation on the distribution of clouds and hazes, on the other hand, is likely to affect even observations of terrestrial planets due to the highly scattering nature of these aerosols and will need to be accounted for in atmospheric retrievals.