Origins and Fates of Close-In Giant Planets

Abstract

The exoplanet census has revealed a diversity of worlds far beyond that of the solar system. This work aims to advance our empirical understanding of the evolutionary processes responsible for this diversity. Chapters 2 and 3 focus on the final tidal inspirals of hot Jupiters. Using transit timing measurements from NASA's Transiting Exoplanet Survey Satellite (TESS), we found that the apparent orbital period of the hot Jupiter WASP-4b is slowly shrinking. Care was warranted however before claiming a detection of tidal orbital decay. In our case, we showed that the data were best explained through a Doppler-like effect caused by an outer planet or brown dwarf; future searches for tidal orbital decay will need to account for similar effects. Chapters 4 through 7 are oriented around the discovery of young short-period giant planets. While thousands of planets have been discovered, most are between one and ten billion years old. These chapters introduce a Cluster Difference Imaging Photometric Survey that uses TESS to find planets around stars in coeval groups younger than one billion years. Most of the targeted stars are in open clusters that are poorly resolved by TESS, so we built our own difference-imaging data reduction pipeline (Chapter 4). Three results based on the data products are highlighted. First, PTFO 8-8695b, a 10 Myr (million-year) old candidate hot Jupiter whose planetary status has been debated for nearly a decade, is not a planet (Chapter 5). The rejection of PTFO 8-8695b implies that the existence of sub-100 Myr old hot Jupiters is an open question. Second, based on TESS transits and ground-based follow-up, the 40 Myr old Saturn-sized TOI 837b is likely a planet (Chapter 6). Its size (8.6 Earth radii) and orbital period (8.3 days) are consistent with a Neptune-mass planet whose rocky core is enveloped by a primordial atmosphere of hydrogen and helium. Finally, based on Gaia kinematics and TESS rotation periods, the open cluster NGC 2516 (150 Myr old) has a halo of stars that spans 500 parsecs tip-to-tip (Chapter 7). This supports a new Gaia-enabled paradigm wherein the halos of open clusters are often more populous than their cores. There are significant implications for upcoming planet searches around young stars.