Our solar system is just one of billions in the Milky Way, situated about half way from the Galaxy's core to its edge, and nestled safely between a pair of spiral arms. Out of those billions of planets, ours is the only one that we know to support life. This begs two questions. First, is our location in the Galaxy especially suitable for life? Second, if we want to find other life out there, where should we focus our search? In this dissertation, I contribute answers to both questions by seeking to better understand the boundaries of the Milky Way's galactic habitable zone (GHZ), the place in the galaxy where habitable worlds are most likely to be found. We start in Chapter 2 by introducing a novel method for finding the average height of surface features on exoplanets, a characteristic that influences a planet's habitability but was heretofore unknowable. We use elevation data for the rocky bodies in our Solar System to simulate their transits in front of stars of different sizes. We provide a relationship between the scatter at the bottom of the resulting light curves and the so-called "bumpiness" of the transiting planet. In Chapter 3, we zoom out from planets to get a better understanding of the dynamical and chemical evolution of the Milky Way, which are both crucial for constraining the Galaxy's GHZ. We use the Extreme Deconvolution Gaussian Mixture Model to identify overdensities of stars in both velocity and action space, called moving groups and orbit groups, respectively. Velocities and actions are calculated using data from the early third data release of the Gaia mission. When we analyze the chemical abundance distributions of these moving and orbit groups with GALAH DR3 data, we find that using velocities alone to define moving groups, or even using velocities and actions together, yields an incomplete view of the underlying density distributions and their origins. Our chemical analysis also confirms expected chemical evolution trends in the Solar neighborhood. Next, we explore the effects of stellar motion and galactic dynamics on the habitability of planets in different regions of the Galaxy. In Chapter 4, we use Gaia DR2 data to calculate 3D galactocentric velocities for stars observed by the Kepler spacecraft. We compare the velocities of confirmed Kepler host stars to those of their non-host stellar twins and find that there's no relationship between stellar velocity and planet occurrence in the Solar neighborhood. In Chapter 5, we shift our attention to the Milky Way bulge, where stars are closer together and moving more quickly on more elliptical orbits than in the disk. We simulate the orbits of bulge stars and use a semi-analytical method to derive the rate of close stellar encounters. We find that roughly 8 in 10 bulge stars will come within 1000 AU of at least 1 other star every billion years. Half of these stars experience dozens of these encounters every gigayear. These encounters can have dramatic consequences for planets, and our findings strongly suggest that the Milky Way bulge is not the most suitable environment for life. In Chapter 6, I share an overview of the science communication and outreach work I've done while in graduate school and explain how it's so closely tied to my research on GHZs.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (p. 166-174).

This thesis investigates how increased global mean temperatures on Earth, induced by the increase in the luminosity of the Sun as it ages, change the types of habitable environments on the planet at local scales over the next 3 Gyr. Rising temperatures enhance silicate weathering rates, reducing atmospheric CO₂ levels to below the threshold for photosynthesis, while simultaneously pushing environments past the temperature tolerances of plant and animal species. This leads to the end of all plant life and animal life (due to reduced food, O₂ and H₂O availability, as well as higher temperatures) within the next 1 Gyr. The reduction in the extent of the remaining microbial biosphere due to increasing temperatures and rapid ocean evaporation is then modelled, incorporating orbital parameter changes until all known types of life become extinct; a maximum of 2.8 Gyr from the present. The biosignatures associated with these changes are determined and the analysis extended to Earth-like extrasolar planets nearing the end of their habitable lifetimes. In particular, the stages in the main sequence evolutions of Sun-like stars within 10 pc are evaluated and used to extrapolate the stage that an Earth-analogue planet would be at in its habitable evolution, to determine the best candidate systems for a far-future Earth-analogue biosphere, highlighting the Beta Canum Venaticorum system as a good target. One of the most promising biosignatures for a microbial biosphere on the far-future Earth (and similar planets) may be CH₄, which could reach levels in the atmosphere that make it more readily detectable than it is for a present-day Earth-like atmosphere. Determining these biosignatures will help expand the search for life to the wider range of environments that will be found as the habitable exoplanet inventory grows and planets are found at different stages in their habitable evolution.