Venus is the most physically similar planet to Earth in the solar system as it is only 5% smaller than Earth, while having 20% less mass. The current surface conditions on Venus are dramatically different than Earth, however climate simulations have shown that Venus may have been able to sustain a temperate climate and surface ocean in its past. While the possibility of an extended habitable period in Venus' history is exciting, it is also alarming since the science community has been unable to reach a consensus on the factors which may have led to Venus' fall from grace. In hopes to uncover the mysteries of Venus' past, an international fleet of orbiters and descent probes will be sent to Venus in the coming decades. These missions will work to constrain Venus' history of water loss and understand the level of past and present geological activity on its surface. In addition to in-situ measurements, we can attempt to learn about the commonality of Venus-like evolutions by studying the plethora of exoplanets discovered in the last few decades. The work presented within this dissertation lays out the groundwork for future observations of exoVenuses with JWST and HWO by documenting known terrestrial planets within the VZ, highlighting those which are optimal targets for JWST, investigating pathways for confidently identifying exoVenuses, and predicting the efficiency at which JWST and HWO can make spectroscopic observations. These topics are discussed throughout the four chapters of this dissertation which are as follows. Chapter 1 is a preliminary study that was conducted in the early phases of the TESS mission to estimate the number of exoVenuses that TESS would discover throughout its primary mission. Chapter 2 is a follow-up study of Chapter 1 which catalogs all of the known exoVenuses and conducts a demographic analysis of known exoVenuses which includes distributions of various planetary, orbital, and stellar parameters. The exoVenuses which should be prioritized for JWST observations are highlighted based on their observability and similarity to Venus. We also evaluate the exoVenuses which do not transit their host star and discuss the potential of directly imaging exoVenuses in the future. Chapter 3 investigates the similarities between exoVenus and exoEarth transmission spectra and identifies pathways by which the two planet types can be differentiated. In addition, JWST observations are simulated for both exoVenuses and exoEarths in order to estimate the observation time needed to detect atmospheric species which can be used to discern the two planets. Lastly, Chapter 4 is focused on identifying ways of determining that an exoEarth is volcanically active through HWO direct imaging observations, and predicting the ability of HWO to detect molecular features in an exoEarth reflectance spectrum. Although this study is not particularly focused on exoVenuses, knowing how to identify active volcanism on exoplanets is applicable to exoVenuses as Venus is predicted to have an abundance of volcanic activity in its history. The main conclusions of the dissertation and potential future work are summarized in Chapter 6.

Exoplanetary science is a highly interdisciplinary field spanning planetary science, astronomy, and geophysics. This thesis presents a comprehensive exploration of exoplanetary science that combines these disciplines to investigate habitability in the context of diverse system architectures and planet sizes. Through interconnected studies, I demonstrate the power of multi-method approaches in discovering exoplanets and exploring the varied nature of exoplanetary systems, testing the boundaries of what can be considered a potentially habitable world.This thesis focuses on the detection and characterization of a wide range of system architectures, including different types of orbits, host stars, and planet sizes. Using transit photometry, radial velocity (RV) measurements, and astrometry I refine orbital parameters and discover planets in both new and known planetary systems. By applying these methods to the HD 9446, HD 43691, and HD 179079 systems, I illustrated the importance of long-term monitoring of planet systems in revealing system architectures. I refined the orbital solutions and transit ephemerides for these systems and discovered evidence of two possible additional long-period companions. Using a combination of RV and astrometry in my study of red giant star ι Draconis allowed me to refine the properties of a known eccentric planet and led to the discovery of a long-period companion on the planet/brown dwarf boundary. While such a massive object itself wouldn’t be considered habitable in the traditional sense, it could potentially host large moons. These moons, if they exist, could provide environments suitable for life, challenging our conventional definition of what constitutes a potentially habitable world and forcing us to broaden our search criteria when looking for places where life might exist. I combined TESS transit photometry and RVs to discover a Saturn-mass planet, TOI-1386 c, that also orbits partially within the HZ, and confirm the presence of a transiting warm sub-Saturn planet, TOI-1386 b. As TOI-1386 b is in the sub-Saturn mass regime this is an important addition to this demographic of planets due to the ongoing debate regarding the existence of the sub-Saturn gap.Further exploring the diverse system architectures that host potentially habitable worlds, I completed a comprehensive catalog of HZ exoplanets, compiling known planets that orbit within or through the HZ. I include the HZ boundaries, percentage of the orbit that each planet spends in their star’s HZ, and observational metrics for each planet such as TSM values and RV amplitude to facilitate selection for future follow-up observations. I highlight the extreme diversity of system environments that could challenge our traditional notions of habitability and call attention to some planets that would be ideal targets to test the boundaries of habitability. This catalog served as the boundary conditions for my final study where I built the Smaller Than Earth Habitability Model (STEHM).Using STEHM I tested the boundaries of habitability by exploring the lower size limit for potentially habitable planets. STEHM is a 1D model based on a thermal interior evolution and CO2 degassing model of a stagnant lid planet combined with a CO2 atmosphere loss module that varies according to the stellar flux received from a Sun-like star over time. By considering various factors, such how internal heat generation, carbon degassing and atmosphere loss changes with the size of the planet, STEHM provides crucial insights for future exoplanet exploration missions and expands our understanding of planetary habitability.As we improve our ability to detect and characterize planets, we uncover an incredible diversity of worlds with different architectures and sizes of planets both in and around the HZ. The techniques and insights presented here will guide the next generation of exoplanet studies of these potentially habitable worlds. Our detection work provides insight into these planetary system architectures, the HZ catalog provides a selection of targets that can serve as the ideal test cases for planetary habitability and our model results from STEHM can be directly applied to future James Webb Space Telescope and HabitableWorlds Observatory observations of smaller than Earth sized planets to help determine their habitability.

The unprecedented contrast ratio of future direct imaging (DI) instruments offers a new pathway to discover and characterize exoplanets around nearby stars. The Nancy Grace Roman Space Telescope, set for launch before May 2027, will conduct a science demonstration mission with its coronagraph instrument to test the new technologies for high-contrast direct imaging of exoplanets. This mission serves as a stepping stone to the Habitable Worlds Observatory, which aims to directly image and characterize Earth-like planets in the habitable zones of nearby stars in the 2040s. To ensure the success of these highly anticipated missions, extensive precursor science is required. This dissertation addresses critical questions for the DI community and demonstrates the role of the radial velocity (RV) method in supporting these goals. I will showcase the RV method as not only a primary discovery tool but also as a unique means to reveal hidden details about planetary systems that other methods cannot access. Then, I will present a method that synergizes RV and DI and explores the parameter space within which planetary signatures are hidden in the RV signal noise due to precision limit but could be detected by DI. The information returned by complementing DI with RV will be crucial to an effective DI target selection process. Next, I will present two planetary discoveries along with a full characterization of a nearby system harnessing the full of power the RV. The result of which provides additional valuable candidates to the DI community. Lastly, I will investigate how observational factors influence the refinement of orbital periods of long-period planets and offer strategies for efficient ground-based RV precursor observations to the DI candidates. These strategies are designed to maximize reductions in planetary orbital location uncertainty using RV within limited time frames, such that DI candidates can be observed at the optimal epochs. The results contained within this dissertation provides key insights and guidelines for precursor science to maximize the science return of future DI missions.

The planetary mass and radius sensitivity of exoplanet discovery capabilities has reached into the terrestrial regime. The focus of such investigations is to search within the Habitable Zone where a modern Earth-like atmosphere may be a viable comparison. However, the detection bias of the transit and radial velocity methods lies close to the host star where the received flux at the planet may push the atmosphere into a runaway greenhouse state. One such exoplanet discovery, Kepler-1649b, receives a similar flux from its star as modern Venus does from the Sun, and so was categorized as a possible exoVenus. Here we discuss the planetary parameters of Kepler-1649b with relation to Venus to establish its potential as a Venus analog. We utilize the general circulation model ROCKE-3D to simulate the evolution of the surface temperature of Kepler-1649b under various assumptions, including relative atmospheric abundances. We show that in all our simulations the atmospheric model rapidly diverges from temperate surface conditions towards a runaway greenhouse with rapidly escalating surface temperatures. We calculate transmission spectra for the evolved atmosphere and discuss these spectra within the context of the James Webb Space Telescope (JWST) Near-Infrared Spectrograph (NIRSpec) capabilities. We thus demonstrate the detectability of the key atmospheric signatures of possible runaway greenhouse transition states and outline the future prospects of characterizing potential Venus analogs.

Current investigations of exoplanet biosignatures have focused on static evidence of life, such as the presence of biogenic gases like O2 or CH4. However, the expected diversity of terrestrial planet atmospheres and the likelihood of both "false positives" and "false negatives" for conventional biosignatures motivate exploration of additional life detection strategies, including time-varying signals. Seasonal variation in atmospheric composition is a biologically modulated phenomenon on Earth that may occur elsewhere because it arises naturally from the interplay between the biosphere and time-variable insolation. The search for seasonality as a biosignature would avoid many assumptions about specific metabolisms and provide an opportunity to directly quantify biological fluxes - allowing us to characterize, rather than simply recognize, biospheres on exoplanets. Despite this potential, there have been no comprehensive studies of seasonality as an exoplanet biosignature. Here, we provide a foundation for further studies by reviewing both biological and abiological controls on the magnitude and detectability of seasonality of atmospheric CO2, CH4, O2, and O3 on Earth. We also consider an example of an inhabited world for which atmospheric seasonality may be the most notable expression of its biosphere. We show that life on a low O2 planet like the weakly oxygenated mid-Proterozoic Earth could be fingerprinted by seasonal variation in O3 as revealed in its UV Hartley-Huggins bands. This example highlights the need for UV capabilities in future direct-imaging telescope missions (e.g., LUVOIR/HabEx) and illustrates the diagnostic importance of studying temporal biosignatures for exoplanet life detection/characterization.

Abstract: Understanding the physical characteristics of Venus, including its atmosphere, interior, and its evolutionary pathway with respect to Earth, remains a vital component for terrestrial planet evolution models and the emergence and/or decline of planetary habitability. A statistical strategy for evaluating the evolutionary pathways of terrestrial planets lies in the atmospheric characterization of exoplanets, where the sample size provides sufficient means for determining required runaway greenhouse conditions. Observations of potential exo-Venuses can help confirm hypotheses about Venus’s past, as well as the occurrence rate of Venus-like planets in other systems. Additionally, the data from future Venus missions, such as DAVINCI, EnVision, and VERITAS, will provide valuable information regarding Venus, and the study of exo-Venuses will be complimentary to these missions. To facilitate studies of exo-Venus candidates, we provide a catalog of all confirmed terrestrial planets in the Venus zone, including transiting and nontransiting cases, and quantify their potential for follow-up observations. We examine the demographics of the exo-Venus population with relation to stellar and planetary properties, such as the planetary radius gap. We highlight specific high-priority exo-Venus targets for follow-up observations, including TOI-2285 b, LTT 1445 A c, TOI-1266 c, LHS 1140 c, and L98–59 d. We also discuss follow-up observations that may yield further insight into the Venus/Earth divergence in atmospheric properties.

Exoplanet hunting efforts have revealed the prevalence of exotic worlds with diverse properties, including Earth-sized bodies, which has fueled our endeavor to search for life beyond the Solar System. Accumulating experiences in astrophysical, chemical, and climatological characterization of uninhabitable planets are paving the way to characterization of potentially habitable planets. In this paper, we review our possibilities and limitations in characterizing temperate terrestrial planets with future observational capabilities through the 2030s and beyond, as a basis of a broad range of discussions on how to advance "astrobiology" with exoplanets. We discuss the observability of not only the proposed biosignature candidates themselves but also of more general planetary properties that provide circumstantial evidence, since the evaluation of any biosignature candidate relies on its context. Characterization of temperate Earth-sized planets in the coming years will focus on those around nearby late-type stars. The James Webb Space Telescope (JWST) and later 30-meter-class ground-based telescopes will empower their chemical investigations. Spectroscopic studies of potentially habitable planets around solar-type stars will likely require a designated spacecraft mission for direct imaging, leveraging technologies that are already being developed and tested as part of the Wide Field InfraRed Survey Telescope (WFIRST) mission. Successful initial characterization of a few nearby targets will be an important touchstone toward a more detailed scrutiny and a larger survey that are envisioned beyond 2030. The broad outlook this paper presents may help develop new observational techniques to detect relevant features as well as frameworks to diagnose planets based on the observables. Key Words: Exoplanets-Biosignatures-Characterization-Planetary atmospheres-Planetary surfaces. Astrobiology 18, 739-778.

Abstract: Stellar X-ray and UV radiation can significantly affect the survival, composition, and long-term evolution of the atmospheres of planets in or near their host star’s habitable zone (HZ). Especially interesting are planetary systems in the solar neighborhood that may host temperate and potentially habitable surface conditions, which may be analyzed by future ground- and space-based direct-imaging surveys for signatures of habitability and life. To advance our understanding of the radiation environment in these systems, we leverage ∼3 Ms of XMM-Newton and Chandra observations in order to measure three fundamental stellar properties at X-ray energies for 57 nearby FGKM stellar systems: the shape of the stellar X-ray spectrum, the luminosity, and the timescales over which the stars vary (e.g., due to flares). These systems possess HZs that will be directly imageable to next-generation telescopes such as the Habitable Worlds Observatory and ground-based Extremely Large Telescopes. We identify 29 stellar systems with L X/L bol ratios similar to (or less than) that of the Sun; any potential planets in the HZs of these stars therefore reside in present-day X-ray radiation environments similar to (or less hostile than) modern Earth, though a broader set of these targets could host habitable planets. An additional 19 stellar systems have been observed with the Swift X-ray Telescope; in total, only ∼30% of potential direct imaging target stars has been observed with XMM-Newton, Chandra, or Swift. The data products from this work (X-ray light curves and spectra) are available via a public Zenodo repository (doi:10.5281/zenodo.11490574).