On The Evolution, Detection, and Characterization of Small Planets in the Habitable Zones of M Dwarfs
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As the technology behind instrumentation in astronomy improves, so too does our ability to detect and characterize worlds outside our solar system. We are currently witnessing a revolution in exoplanet science: for the past three decades, the number of known planets orbiting other stars has grown exponentially, showing no signs of tapering off. We now know of dozens of small planets in the habitable zones of their stars, and this number is expected to grow with upcoming survey missions such as the Transiting Exoplanet Survey Satellite (TESS) and the PLAnetary Transits and Oscillations telescope (PLATO). Improving commensurately with our capacity to detect these planets is our ability to characterize them. Missions such as the James Webb Space Telescope (JWST) and subsequent generations of space-based telescopes will be capable of characterizing these planets' atmospheres and searching for molecular signatures of habitability and life. Given the large number of potentially habitable planets we will soon discover, knowing which targets to prioritize for follow-up observations is paramount to furthering our goal of understanding the potential for habitability of exoplanets. Once data becomes available, its interpretation will rely heavily on a physical understanding of the processes that contribute to making a planet habitable (or not). Models of the evolutionary processes of potentially habitable planets can therefore improve target selection for biosignature searches and enhance the science return from terrestrial planet characterization. In this dissertation, I develop theoretical models of the evolution of the atmospheres and surface water inventories of planets in the habitable zones of low mass stars. While these stars currently offer the best opportunity to characterize potentially habitable planets, my work shows that vigorous atmospheric escape from these planets due to intense stellar activity could render many of them uninhabitable. I discuss observational signatures of the escape process and best case scenarios for planets around low mass stars, including the possibility that planets that form with substantial primordial atmospheres of hydrogen and helium could weather the active phase of the host star without substantial devolatilization. I also refine existing techniques to detect and characterize exoplanets, with particular emphasis on small planets in the habitable zones of low mass stars. I introduce EVEREST, a pipeline to remove instrumental noise from photometric datasets and enable the detection of planet transit signals that would otherwise be hidden in the noise. Furthermore, I develop two novel techniques for the detection and characterization of potentially habitable exoplanets: the exo-auroral method, which relies on the spectroscopic detection of auroral emission from terrestrial planets, and planet-planet occultations, wherein an exoplanet occults another planet in the same system, imparting a small photometric signal on the system's light curve. I show how the next generation of telescopes may enable the application of both techniques to planets in the habitable zones of low mass stars, uncovering detailed information about their orbits and surface/atmospheric properties. I discuss all of my results in the context of TRAPPIST-1, a nearby low mass star hosting seven transiting planets, three of which are in the habitable zone. This and similar soon-to-be discovered systems will likely revolutionize our understanding of exoplanets, habitability, and astrobiology in general.
- Astronomy