Discovery and Demographics of Hot Planets Orbiting Hot Stars

Surveys dedicated to detecting exoplanets via the transit and radial velocity methods have revolutionized our understanding of planet formation and evolution by revealing the the prevalence of planets orbiting close to their stars. Transit surveys have been especially groundbreaking due to their abilities to discover large quantities of planets with small orbital separations, which can they be further characterized via transmission spectroscopy, emission spectroscopy, and thermal phase curve observations to reveal details about their atmospheres and surfaces. In recent years, the Transiting Exoplanet Survey Satellite (TESS) has provided the opportunity to expand these techniques into entirely new regimes, due to its ability to search for transiting planets around a greater variety of stars and around brighter stars that are more amendable to follow-up observations. This thesis focuses on the utilization of TESS data to search for and study the demographics of these planets./nFirst, I present TRICERATOPS, a tool designed to statistically validate transiting exoplanets and identify likely astrophysical false positives in TESS data. To statistically validate a transiting exoplanet is to confirm its planetary nature by ruling out plausible false positive scenarios, such as those that arise when multiple stars are blended together in the data. This is a particularly pertinent problem for TESS, which is equipped with relatively low-resolution cameras that often cannot distinguish light originating from individual stars, especially in crowded fields. I discuss the design and efficacy of TRICERATOPS, demonstrating that it is an effective tool for identifying the most promising planet candidates detected by TESS and prioritizing follow-up observations with both ground-based and space-based telescopes./nNext, I use TRICERATOPS and an array of ground-based follow-up observations to validate 13 hot and potentially terrestrial planets detected by TESS. These planets are unlike any rocky bodies in the Solar System; they orbit their stars at distances of only a few stellar radii and are so highly irradiated that many are expected to have molten surfaces. Their high temperatures also mean that they emit infrared light at levels detectable by JWST. Emission spectroscopy and thermal phase curve observations of these worlds can reveal the presence and composition of an atmosphere, measure Bond albedo, and calculate heat redistribution properties. Prior to TESS, very few of these types of planets were known around bright stars amenable to JWST observations. This sample therefore facilitates the investigation of hot Earth-size worlds at a population level./nFinally, I conduct a search for planets smaller than Saturn orbiting A-type stars. A-type stars, which are roughly twice as massive and nearly twice as hot as Sun-like stars, have historically been avoided by transit and radial velocity surveys due to their large radii and rapid rotation rates, which hinder our ability to detect planets around them. As a consequence, early transit surveys like the Kepler mission acquired very little data of these stars, limiting our understanding of planet demographics to stars like the Sun and cooler. By observing all bright stars across nearly the entire sky, TESS has provided the best opportunity yet to search for small planets orbiting relatively hot stars. Through this search, I discover and validate a single planet: HD 56414 b, a Neptune-size planet orbiting one of the hottest planet-hosting stars known to date on a 29-day orbital period. The orbital period of this planet is long compared to the typical planet detected by TESS, suggesting that Neptune-size planets with smaller orbital separations may not exist around A-type stars. I display that atmospheric photoevaporation due to high levels of near-ultraviolet radiation offers one possible explanation for this phenomenon./nTo test this hypothesis more robustly, I calculate the occurrence rate of small planets with orbital periods under 10 days around A-type stars. I demonstrate, for the first time, that sub-Saturns and sub-Neptunes are rarer around A-type stars than they are around their cooler counterparts. I also find evidence that super-Earths are as common or less common around A-type stars than cooler stars. These results suggest that small planets are unable to form at, migrate to, or survive at the small orbital separations probed by TESS around these hot stars. Overall, these findings significantly advance our understanding of how planets form and evolve around stars hotter than the Sun, providing a more holistic picture of planetary populations throughout the galaxy.

Advisor: Dressing, Courtney D