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.

This dissertation is an exploration of the effects of the collision of large asteroids and comets on the atmosphere of the Earth and Earthlike planets. The first task undertaken is the characterization of the impact rates in the inner solar system during the present time, and during the first billion years of Solar System history when the flux was changing rapidly. Once defined, these fluxes are used to model the long term cumulative effect of multiple impacts on planetary atmospheres. The implications of cometary impacts on evolution of the water and deuterium abundances on Venus are examined. The short lifetime of water on Venus suggests that the water abundance is in a quasi-steady-state balance between loss by escape and replenishment by infall. In addition, the observed deuterium-to-hydrogen ratio on Venus is consistent with a steady state and does not necessarily imply a past water excess. Results are presented of a model incorporating a stochastic cometary source and nonthermal escape of hydrogen that produces the observed water abundance and D/H ratio. The stochastic variability of each of these quantities is shown to be large. Water on Venus is likely to be in a near steady state mediated by large comet impacts. The early history of water on the planet has been obscured by a history of random impacts. A study of the effects of impact-generated dust clouds on the primitive Earth leads to the conclusion that such clouds were significant perturbers of the early climate. The Earth was shrouded by an optically-thick dust cloud for ≈ 150-250 m.y. During this time the surface temperature was equal to the planetary equilibrium temperature unless significant heating by impacts or surface heat flow existed beneath the dust cloud. An admixture of a few per cent of organic materials in the cloud may have significantly lowered the planetary bond albedo, thereby raising the equilibrium temperature. The epoch of continuous dust shrouding was followed by a period of stochastically intermittent dust clouds occuring at greater intervals as the early intense bombardment subsided towards the present day flux.

The Mission Accessible Near-Earth Objects Survey aims to physically characterize sub-km near-Earth objects (NEOs). We report the first photometric results from the survey that began in 2013 August. Photometric observations were performed using 1-4 m class telescopes around the world. We present rotational periods and light curve amplitudes for 86 sub-km NEOs, though in some cases only lower limits are provided. Our main goal is to obtain light curves for small NEOs (typically, sub-km objects) and estimate their rotational periods, light curve amplitudes, and shapes. These properties are used for a statistical study to constrain overall properties of the NEO population. A weak correlation seems to indicate that smaller objects are more spherical than larger ones. We also report seven NEOs that are fully characterized (light curve and visible spectra) as the most suitable candidates for a future human or robotic mission. Viable mission targets are objects fully characterized, with Delta v(NHATS) <= 12 km s(-1), and a rotational period P > 1 hr. Assuming a similar rate of object characterization as reported in this paper, approximately 1230 NEOs need to be characterized in order to find 100 viable mission targets.

As followup to our recent detection of a pre-transit signal around HD 189733 b, we obtained full pre-transit phase coverage of a single planetary transit. The pre-transit signal is again detected in the Balmer lines but with variable strength and timing, suggesting that the bow shock geometry reported in our previous work does not describe the signal from the latest transit. We also demonstrate the use of the Ca II H and K residual core flux as a proxy for the stellar activity level throughout the transit. A moderate trend is found between the pre-transit absorption signal in the 2013 data and the Ca II H flux. This suggests that some of the 2013 pre-transit hydrogen absorption can be attributed to varying stellar activity levels. A very weak correlation is found between the Ca II H core flux and the Balmer line absorption in the 2015 transit, hinting at a smaller contribution from stellar activity compared to the 2013 transit. We simulate how varying stellar activity levels can produce changes in the Balmer line transmission spectra. These simulations show that the strength of the 2013 and 2015 pre-transit signals can be reproduced by stellar variability. If the pre-transit signature is attributed to circumplanetary material, its evolution in time can be described by accretion clumps spiraling toward the star, although this interpretation has serious limitations. Further high-cadence monitoring at H alpha is necessary to distinguish between true absorption by transiting material and short-term variations in the stellar activity level.

In order to search for evidence of hydration on M-type asteroid (16) Psyche, we observed this object in the 3 mu m spectral region using the long-wavelength cross-dispersed (LXD: 1.9-4.2 mu m) mode of the SpeX spectrograph/imager at the NASA Infrared Telescope Facility. Our observations show that Psyche exhibits a 3 mu m absorption feature, attributed to water or hydroxyl. The 3 m absorption feature is consistent with the hydration features found on the surfaces of water-rich asteroids, attributed to OH- and/or H2O-bearing phases (phyllosilicates). The detection of a 3 mu m hydration absorption band on Psyche suggests that this asteroid may not be a. metallic core, or it could be a metallic core that has been impacted by carbonaceous material over the past 4.5 Gyr. Our results also indicate rotational spectral variations, which we suggest reflect heterogeneity in the metal/silicate ratio on the surface of Psyche.

Internal gravity waves are a common feature of stratified fluids. They facilitate transport of momentum and energy – thus influencing the evolution of the fluid. There is a large body of research addressing the behaviour of gravity waves in the terrestrial atmosphere. This thesis builds and extends the research to giant planets – in particular to close-in extrasolar giant planets and the solar system giant planet, Jupiter. Because the atmospheres of close-in giant planets are expected to be strongly stratified, knowledge of the behaviour of gravity waves in such atmospheres is especially important. Close-in giant planets are thought to have their rotations and orbital period 1:1 synchronised, i.e., they are “tidally locked”. Such planets do not exist in the Solar System. However, many are known from observations of extrasolar systems. Their synchronisation means that they have a permanent day-side and night-side leading to interesting atmospheric dynamics. Modelling these circulations with global circulation models (GCMs) and comparing these models with observations is an active research area. However, many GCMs filter some or all gravity waves removing their effects. This thesis addresses this by explicitly looking at the effects gravity waves can have on the circulation. It is shown that gravity waves provide a mechanism for accelerating, decelerating, and heating the flow. Further, horizontally propagating gravity waves are shown to provide a possible means for coupling the day- and night-sides of tidally locked planets. As well as affecting the dynamics of the atmosphere, gravity wave behaviour is affected by the dynamics of the atmosphere. Therefore, gravity waves can be used to explore atmospheric properties. In this thesis gravity waves observed in Jupiter’s atmosphere, by the Galileo probe, are used to identify features of Jupiter’s atmosphere such as the altitude of the turbopause and the vertical profile of zonal winds at the probe entry site.

The detection of more than 130 multiple planet systems makes it necessary to interpret a broader range of properties than are shown by our Solar system. This thesis covers aspects linked to the proliferation in recent years of multiple extrasolar planet systems. A narrow observational window, only partially covering the longest orbital period, can lead to solutions representing unrealistic scenarios. The best-fit solution for the three-planet extrasolar system of HD 181433 describes a highly unstable configuration. Taking into account the dynamical stability as an additional observable while interpreting the RV data, I have analysed the phase space in the neighbourhood of the statistical best-fit. The two giant companions are found to be locked in the 5:2 MMR in the stable best-fit model. I have analysed the dynamics of the system HD 181433 by assessing different scenarios that may explain the origin of these eccentric orbits, with particular focus on the innermost body. A scenario is considered in which the system previously contained an additional giant planet that was ejected during a period of dynamical instability among the planets. Also considered is a scenario in which the spin-down of the central star causes the system to pass through secular resonance. In its simplest form this latter scenario fails to produce the system observed. If additional short-period low mass planets are present in the system, I find that mutual scattering can release planet b from the secular resonance, leading to a system with orbital parameters similar to those observed today. Finally, I have studied the evolution of low mass planets interacting with a gas-giant planet embedded in a gaseous disc. The transit timing method allows the detection of non-transiting planets through their gravitational perturbations. I have investigated the detectability of low mass planets neighbouring short-period giants after protoplanetary disc dispersal.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (p. 85-89).

Thesis (Ph. D.)--University of Missouri-Columbia, 1999. / Typescript. Vita. Includes bibliographical references (leaves 116-120). Also available on the Internet.

I report the results of a three-year intensive radial-velocity survey of 22 planet-host stars in search of the low-amplitude (K ~5-10 m s⁻¹) signals from additional planets which may be "hiding" in the residuals of the known planet orbital solution. On average, more than 40 radial-velocity observations were obtained for each target using the High-Resolution Spectrograph at the 9.2m Hobby-Eberly Telescope (HET). These high-precision data can be used to rule out additional planets in some of these systems to a detection limit of M sin i ~10-20 Earth masses at a = 0:05 AU. Jupiter-mass planets can be excluded at the 99% level for orbital separations a < 2 AU. No additional planets are evident, and our data do not confirm the planets HD 20367b, HD 74156d, and 47 UMa c. Test particle simulations of these systems with the SWIFT N-body integrator reveal the regions where additional planets could reside in stable orbits. Further simulations with Saturn-mass bodies in these regions are also performed. We note a lack of short-period giant planets in any of these 22 systems, despite dynamical feasibility. The frequency of inner giant planets may be much lower than what was expected based on early discoveries of such objects in systems containing jovian-mass planets. Terrestrial-mass planets may be present in these systems but as yet undetectable. These results suggest that planet formation and migration processes do not favor systems containing both "hot" and "cold" Jupiters. Hence, as detection methods become sensitive to terrestrial-mass planets, systems with architectures like our own Solar system may yet be commonplace. / text