Stellar evolution and giant exoplanets

Abstract

The fate of a planet is deeply influenced by the evolution of the star it orbits. With the sample of confirmed exoplanets nearing 6,000, we are now positioned to examine how stellar evolution affects the structure and dynamics of planetary systems across their full lifetimes. While many studies focus on main-sequence systems, planets orbiting evolved stars provide a unique lens into planetary evolution. In this dissertation, I conduct a focused investigation of exoplanets orbiting subgiant and red giant stars, coupling observations of their planetary architectures to changes in their host stars' internal structure. Using data from NASA's Transiting Exoplanet Survey Satellite, I developed and applied a specialized open-source pipeline to identify and confirm transiting planets orbiting evolved stars. This effort has so far led to the discovery and confirmation of fifteen new planets, including five detailed in this dissertation. The orbital architectures of these systems provide the first empirical evidence for post-main-sequence realignment between the orbital planes of short-period giant planets and the stellar spin axes of their hosts. These stars, originally hot and likely misaligned on the main sequence, have since cooled and developed convective envelopes, allowing tidal dissipation to realign their spins, marking a previously unobserved mechanism in planetary system evolution. Beyond orbital dynamics, these evolved systems also test key predictions for planet interior physics. Some of the planets discovered in this work appear inflated, as expected from models of post-main-sequence irradiation and energy deposition. However, others are not inflated despite receiving comparable levels of incident flux. A comparative analysis of their orbital eccentricities and radii supports the hypothesis that tidal heating during orbital circularization may play a critical role in radius inflation. To contextualize these planets within their host stars' evolutionary histories, I also investigate stellar spin-down as a probe of age. While gyrochronology offers precise age estimates for cool main-sequence stars, older stars are known to deviate from standard rotational evolution due to weakened magnetic braking. To model this effect, I trained a neural network emulator for stellar rotational evolution and used it to perform a population-level analysis of Kepler stars with asteroseismically measured rotation periods and ages. I find that deviations from standard spin-down begin slightly before solar age, and that while current weakened braking models improve fits to data, they remain incomplete beyond the age of the sun. Together, these projects demonstrate that evolved stars are powerful laboratories for understanding the long-term evolution of planetary systems. By combining new exoplanet discoveries, orbital dynamics, asteroseismic stellar characterization, and rotational modeling, this dissertation places new observational and theoretical constraints on key processes shaping planetary lifetimes.