UC Berkeley

The origin and evolution of our Solar System, our home planet, and our species are mysteries that depend on a rich, complex history of stochastic formation processes. The discovery of planets orbiting other stars beyond the Solar System challenged canonical wisdom on how and when these processes might have unfolded, signaling a paradigm shift in how we see the Earth, life, and ourselves in relation to the Cosmos. The past 30 yr of exoplanet discovery have continued to reveal an overwhelming diversity of planetary properties, host stars, and environments that are completely unlike the Solar System, leading to more questions than answers in how planets and life may emerge. In this dissertation, I present theoretical simulations and multi-technique observations of exoplanets and their host stars to investigate of how planetary systems similar to ours form, evolve, and become habitable. First, I report on the first investigation of potentially habitable exomoons in the HIP 41378 multi-planet system. I present astroQTpy, an open-source Python package that uses a quadtree framework to accelerate N-body experiments for hierarchical star-planet-moon systems. Applying this package to the planet HIP 41378 f, a temperate Jovian in the optimistic habitable zone of its host star, I simulate plausible exomoon properties and orbits that are consistent with space-based transit observations from the K2 mission and Hubble Space Telescope (HST). Combining these simulations with tidal evolution simulations and photodynamical transit models, I show that long-term stable, Earth-sized moons around HIP 41378 f are dynamically viable and consistent with observations. Therefore, this planet is a promising laboratory for testing theories of exomoon formation and habitability in the era of extremely-high-precision photometry with the James Webb Space Telescope (JWST) and PLATO (PLAnetary Transits and Oscillations of stars). Next, I discuss ways in which stars are interconnected with planetary habitability and present a catalog of key stellar parameters of nearby stars. I analyze the FUV--MIR spectral energy distributions of nearby FGKM stars using Bayesian model averaging to derive robust stellar parameters (e.g., effective temperature, luminosity, etc.) that are relevant to planetary habitability. I present a public catalog that integrates these stellar parameters with flare rates and optical variability from the Transiting Exoplanet Survey Satellite (TESS), X-ray fluxes, and stellar abundance from the literature. This critical dataset informs the selection of high-priority targets for future exoplanet direct-imaging missions like the Habitable Worlds Observatory (HWO) by providing essential inputs for modeling rocky planet interiors, out-gassed atmospheres, and global biosignatures. Then, I present a deep archival radial velocity (RV) search of >100 nearby Sun-like stars to explore the range of planetary architectures and evolutionary pathways where habitable exoplanets could exist. I discuss the inception and outcomes of a community science project involving >60 participants, which assembled over 150,000 archival RV measurements from 23 spectrographs spanning 36 yr to constrain companion masses and orbits, identify new Keplerian signals, and measure sensitivity as a function of minimum mass and semi-major axis. The community science project (called the "SPORES-HWO RV Hunters") revealed that current RV data remain insensitive to Jupiter-like giant analogs in about 50% of these nearby systems and typically probe only planets more massive than approximately 50 Earth masses in the habitable zone. This motivates additional long-baseline RV monitoring to constrain the architectures of these systems, identify dynamically disruptive companions, and provide planetary system context that will enable future searches for habitable worlds with space-based direct imaging. Finally, I present preliminary results from new RV observations targeting nearby Sun-like stars previously overlooked in exoplanet surveys. I discuss new evidence for at least one eccentric giant companion orbiting a high-priority HWO direct-imaging target in the habitable zone. Such giant planets may be dynamically incompatible with Earth-like habitable zone planets, and are therefore necessary to constrain prior to direct-imaging exoEarth surveys to maximize efficiency. This potential planet discovery demonstrates how systematic follow-up of neglected systems continues to uncover key missing pieces of the local planetary census.