In the next few years, the launch of the James Webb Space Telescope (JWST), along with the construction of new ground-based observatories, will provide the opportunity to attempt atmospheric characterization of terrestrial planets in the habitable zones of nearby M dwarf stars. For the first time, the assessment of habitability and the possibility of detecting biosignatures from planets around other stars will be within the capabilities of astronomical observatories. Truly Earth-like planets (i.e. orbiting a Sun-like, G-type star) are not yet accessible, and may not be until the selection, construction, and launch of a next-generation space telescope, such as LUVOIR or HabEx, which are under consideration for potential prioritization by the 2020 Decadal Survey on Astronomy and Astrophysics. In the immediate future, it will only be possible to characterize the atmospheres of Earth-sized planets that orbit M dwarf hosts, because the methods of observation for imminent observatories favor shorter-period planets and stronger signal can be achieved with smaller star-planet size ratios. However, planets orbiting M dwarfs face an evolutionary history very different than a planet like Earth, orbiting a Sun-like, G-type star. Additionally, these stars go through a much longer superluminous pre-main-sequence phase than G dwarfs, which can drive ocean loss via the runaway greenhouse effect, subsequent photolysis from stellar UV radiation, and, finally, permanent loss of hydrogen to space. As a result, M dwarf habitable zone planets can be stripped of their volatiles before life could originate and proliferate. Even if life did originate, M dwarf stars generally exhibit intense levels of high-energy activity throughout their main-sequence lifetimes, so the planetary surface can experience much more extreme irradiation than the early Earth environment. Additionally, because M dwarfs are small and dim, planets must orbit much closer to the star than Earth does to the Sun to allow for the possibility of liquid water on their surfaces. This proximity increases the probability for such planets to be synchronously rotating with their host star, which may result in large temperature differences between the permanent day and permanent night sides, raising the possibility of atmospheric collapse on the night side of the planet. Despite these challenges, the observational advantages of M dwarf stars mean that they will be the first place to search for habitability and life outside the Solar System. Several small planets have recently been discovered in the habitable zones around nearby M dwarf stars during ground-based surveys (e.g. TRAPPIST, MEarth, and HARPS). Of these, I focus on the TRAPPIST-1 planetary system, whose seven Earth-sized planets provide an unprecedented opportunity to study planetary evolution and habitability in a single system, which includes three planets in the traditional habitable zone. As TRAPPIST-1 is an ultra-cool dwarf star (spectral type M8V), its planets are more easily amenable to near-term observations compared to other terrestrial-sized planet discoveries around earlier-type stars (e.g. LHS 1140 b and c, Ross 128 b), because of the exceptionally diminutive size of the TRAPPIST-1 star (barely larger than Jupiter), maximizing the planet-to-star signal. To support upcoming observations of nearby M dwarf planetary systems, I provide foundational modeling efforts to understand the range of likely environmental states of the TRAPPIST-1 planets and how to spectrally discriminate them. I developed a versatile, coupled climate-photochemical model for terrestrial exoplanets. Using this model, I present self-consistent climate-photochemical model atmospheres of a wide range of potential TRAPPIST-1 planetary states, and generate and analyze synthetic spectra of these planets to identify observational features that can be used to distinguish between these planetary environmental outcomes. The modeled planetary states span evolved, post-runaway, desiccated planets with thick atmospheres, to a variety of water worlds. To assess a variety of environments that could be possible using a robust radiative transfer model, but also consider the day-night differences these planets may experience, I develop a two-column, day-night mode for an advanced 1D radiative-convective climate model, VPL Climate. The diversity of possible environments modeled here supports the habitable zone as probabilistic: encompassing a range of possible states for each planet, which may or may not be habitable. Planets within the habitable zone could be either freezing, temperate, or hot, depending on their atmospheric composition. Planets beyond the outer edge, such as TRAPPIST-1 h, could also have temperate or hot atmospheres, if they have a Venus-like greenhouse effect. Potential observational discriminants for these atmospheres in transmission and emission spectra are influenced by photochemical processes and aerosol formation. The atmospheric states simulated here include collision-induced oxygen absorption (O2-O2), and O3, CO, SO2, and H2O absorption features, with transit signals of up to 200 ppm, well above the 20-30 ppm putative noise floor of JWST in the NIR. These simulated transmission spectra are consistent with K2, Hubble Space Telescope, and Spitzer Space Telescope observations of the TRAPPIST-1 planets. To help discriminate ambiguous observations, including the detection of water vapor, I assess the possibility of detecting isotopic evidence for ocean loss in transit transmission spectra. In the Solar System, differences in isotopic abundances between the Solar abundance and planetary atmospheres have been used to infer the history of ocean loss and atmospheric escape (e.g. Venus, Mars). I show that H2O and CO2 isotopologues could similarly be used as indicators of past ocean loss and atmospheric escape of terrestrial planets around M dwarfs. These measurements may be possible with JWST if the escape mechanisms and resulting isotopic fractionation were similar to Venus, but exist in a more transparent atmosphere, such as N2-dominated, or an O2-dominated atmosphere that may result from extreme water loss. In these atmospheres, isotopologue bands are detectable throughout the near-infrared (1-8 [micro]m), especially 3-4 [micro]m. These are not likely detectable in CO2-dominated atmospheres because the saturated CO2 bands obscure key HDO features, and at the high temperatures exhibited by a Venus-like atmosphere, the ro-vibrational quantum states of the rare isotopologues are not occupied. The results of spectral modeling suggest that the detection of O2-O2 along with increased fractionation in HDO relative to Earth would be strong evidence that a planet is not habitable, despite detections of atmospheric oxygen and water, which would normally be considered evidence of an inhabited Earth-like world. The results of this dissertation have demonstrated a small but diverse selection of plausible planetary conditions given current knowledge of planetary processes that may exist on other worlds, which nonetheless have provided a broad exploration of environmental states for the TRAPPIST-1 planets. The combined studies point to multiple spectral discriminants to identify past ocean loss and to differentiate between different environmental states. Although spatially-resolved models (from two columns to full 3D GCMs) can assess the climate distribution on a planet, transit transmission spectra are most sensitive to regions of the atmosphere where temperature gradients are usually small, and where the primary processes are radiation and photochemistry, a regime ideally suited to 1D coupled climate-photochemical models. The spectral discriminants presented here and in future work will help guide and interpret upcoming observations of planets in and around the habitable zones of M dwarf stars, particularly the TRAPPIST-1 system, which is already scheduled for observation with JWST.

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.

The past decade has delivered remarkable discoveries in the study of exoplanets. Hand-in-hand with these advances, a theoretical understanding of the myriad of processes that dictate the formation and evolution of planets has matured, spurred on by the avalanche of unexpected discoveries. Appreciation of the factors that make a planet hospitable to life has grown in sophistication, as has understanding of the context for biosignatures, the remotely detectable aspects of a planet's atmosphere or surface that reveal the presence of life. Exoplanet Science Strategy highlights strategic priorities for large, coordinated efforts that will support the scientific goals of the broad exoplanet science community. This report outlines a strategic plan that will answer lingering questions through a combination of large, ambitious community-supported efforts and support for diverse, creative, community-driven investigator research.

Are we alone in the universe? How did life arise on our planet? How do we search for life beyond Earth? These profound questions excite and intrigue broad cross sections of science and society. Answering these questions is the province of the emerging, strongly interdisciplinary field of astrobiology. Life is inextricably tied to the formation, chemistry, and evolution of its host world, and multidisciplinary studies of solar system worlds can provide key insights into processes that govern planetary habitability, informing the search for life in our solar system and beyond. Planetary Astrobiology brings together current knowledge across astronomy, biology, geology, physics, chemistry, and related fields, and considers the synergies between studies of solar systems and exoplanets to identify the path needed to advance the exploration of these profound questions. Planetary Astrobiology represents the combined efforts of more than seventy-five international experts consolidated into twenty chapters and provides an accessible, interdisciplinary gateway for new students and seasoned researchers who wish to learn more about this expanding field. Readers are brought to the frontiers of knowledge in astrobiology via results from the exploration of our own solar system and exoplanetary systems. The overarching goal of Planetary Astrobiology is to enhance and broaden the development of an interdisciplinary approach across the astrobiology, planetary science, and exoplanet communities, enabling a new era of comparative planetology that encompasses conditions and processes for the emergence, evolution, and detection of life.

Over the past twenty years, astronomers have identified hundreds of extrasolar planets--planets orbiting stars other than the sun. Recent research in this burgeoning field has made it possible to observe and measure the atmospheres of these exoplanets. This is the first textbook to describe the basic physical processes--including radiative transfer, molecular absorption, and chemical processes--common to all planetary atmospheres, as well as the transit, eclipse, and thermal phase variation observations that are unique to exoplanets. In each chapter, Sara Seager offers a conceptual introduction, examples that combine the relevant physics equations with real data, and exercises. Topics range from foundational knowledge, such as the origin of atmospheric composition and planetary spectra, to more advanced concepts, such as solutions to the radiative transfer equation, polarization, and molecular and condensate opacities. Since planets vary widely in their atmospheric properties, Seager emphasizes the major physical processes that govern all planetary atmospheres. Moving from first principles to cutting-edge research, Exoplanet Atmospheres is an ideal resource for students and researchers in astronomy and earth sciences, one that will help prepare them for the next generation of planetary science. The first textbook to describe exoplanet atmospheres Illustrates concepts using examples grounded in real data Provides a step-by-step guide to understanding the structure and emergent spectrum of a planetary atmosphere Includes exercises for students

Thanks to the observation of a growing number of planetary atmospheres, we are at the dawn of a major scientific revolution in atmospheric and climate sciences. But are we ready to understand what will be discovered around other stars? This book brings together 15 review chapters that study and provide up-to-date information on the physical and chemical processes that control the nature of atmospheres. It identifies commonalities between various solar system atmospheres, analyzes the dynamic processes behind different atmospheric circulation regimes, and outlines key questions remaining in solar system science. Through this comprehensive overview, the volume will help researchers understand the possible nature of the exo-atmospheres to be discovered in the coming decades thanks to upcoming new generations of telescopes. Previously published in Space Science Reviews in the Topical Collection "Understanding the Diversity of Planetary Atmospheres”

Based on the author’s own work and results obtained by international teams he coordinated, this SpringerBrief offers a concise discussion of the origin and early evolution of atmospheres of terrestrial planets during the active phase of their host stars, as well as of the environmental conditions which are necessary in order for planets like the Earth to obtain N_2-rich atmospheres. Possible thermal and non-thermal atmospheric escape processes are discussed in a comparative way between the planets in the Solar System and exoplanets. Lastly, a hypothesis for how to test and study the discussed atmosphere evolution theories using future UV transit observations of terrestrial exoplanets within the orbits of dwarf stars is presented.