A multi-parameter approach to exoplanet habitability
Upcoming telescopes will be able to remotely detect potential biosignatures in exoplanet atmospheres and discover signs of life beyond our Solar System. To make the most of the limited observational resources available, target selection has focused on 'habitable worlds' defined as rocky bodies orbiting their host stars at a distance where stellar radiation is suitable for the presence of surface liquid water and enough surface gravity to sustain an atmosphere. With the ever-increasing number of detected exoplanets, we might end up with hundreds of planets that suit these criteria and are accordingly all equally likely to host life. Therefore, we must rethink our classification of what makes a planet habitable. This thesis investigates multiple factors such as planetary magnetism, tectonic activity, surface pressure, and surface temperature that affect habitability. Evidence from the solar system suggests that, unlike Venus and Mars, the presence of a significant magnetic dipole moment and the maintenance of a long-term carbon cycle aided by plate tectonics has assisted in preserving liquid water on Earth's surface. Modelling the magnetic dipole moments of detected rocky exoplanets reveals that 44% +/- 13% of the planets in the circumstellar habitable zone have a negligible magnetic dipole moment. Subsequently, an investigation into tidally induced tectonic activity on the detected rocky exoplanets suggests that 57% +/- 14% of the planets in the circumstellar habitable zone reside within the optimum regime for internal heating, with 14% +/- 2% likely to have tidally driven mobile lid tectonic activity that would help in maintaining the presence of surface liquid water and could be comparable to plate tectonics on Earth. The research conducted on magnetism, tectonics, and tides has revealed many Earth-centric assumptions and biases that still exist in current models. Thus, I develop a primary classification system to characterise the detected rocky exoplanets based on their closest solar system analogue to more accurately infer information regarding atmospheric surface pressure and temperature as they relate to exoplanet habitability. Applying this classification method to the detected rocky exoplanets reveals that 2% +/- 1% are Earth analogues, and an additional 11% +/- 1% of Venus analogues are likely to have significant atmospheres that could be observed in future spectroscopic and photometric missions searching for biosignatures. Furthermore, applying the new classification system, I plot an analogue adjusted surface pressure-temperature phase diagram and predict the potential composition of liquid, ice, or gas on six exoplanets located within the Circumstellar Habitable Zone. This thesis provides a starting point for the application of the limited amount of observational data currently available to the postulate that planetary and astronomical factors such as magnetism, tectonic activity, tidal locking, and atmospheric and surface features play an important role in establishing suitable conditions for the maintenance of surface liquid water on exoplanets. Expanding to a multi-parameter approach to habitability by analysing, modelling, and determining how multiple factors interact on any given planetary body, will enable us to prioritise future observations of planets most likely to maintain liquid water. In conjunction with the rapidly increasing information from exoplanet surveys and databases, the research presented in this thesis will help determine optimal targets for near-future ground- and space-based observations of planetary atmospheres and surfaces.
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