Our ability to detect and characterize small planets in diverse environments is expanding rapidly with the development and continued improvement of the transit and radial velocity methods. Better models, instruments, and telescopes are producing greater planet yields and tighter planetary radius and mass constraints, which in turn provide new targets for atmospheric characterization and produce new insights on planet composition, formation, and evolution. In this thesis, I present work on the characterization and mass determination of small planets with the radial velocity method, the detection of new planets via the transit method, and the study of a planet’s atmosphere through transmission spectroscopy and its implications for planet formation and planet population features.

First, I report on mass estimation and characterization of the long-period exoplanet Kepler- 538b. This sub-Neptune with a period of P = 81.7 days is the only planet known to be orbiting its Sun-like star (0.892 M⊙). Simultaneously modeling Kepler photometry and radial velocities (RVs) yields a semi-amplitude of 1.68 ± 0.39 m s−1 and a planet mass of 10.6 ± 2.5 M⊕, which made Kepler-538b the smallest planet beyond P = 50 days with an RV mass measurement at the time of publication. Precise mass measurements on long-period planets will not only directly address questions about the long-period planet population, but also draw comparisons and shed light on aspects of the short-period planet population like the planetary radius occurrence gap and the impact of high stellar irradiation on exoplanet compositions and atmospheres.

Next, I discuss K2-136c, a sub-Neptune with a period of P = 17.3 days and the largest of three transiting planets orbiting a late-K dwarf (0.742 M⊙) in the young Hyades open cluster (650 ± 70 Myr). Collecting and analyzing RV data from the HARPS-N and ESPRESSO spectrographs jointly with photometry from the K2 and TESS space telescopes yielded an RV semi-amplitude of 5.46 ± 0.45 m s−1 for K2-136c, corresponding to a mass of 18.0 ± 1.7 M⊕. K2-136c is now the smallest planet to have a measured mass in an open cluster and one of the youngest planets ever with a mass measurement. As a result, this system adds an important new window into young small planet compositions, atmospheric mass loss constraints around young active stars, and planetary evolution at relatively unexplored ages.

I then present the TATER planet detection pipeline and apply it to high-cadence photometry of 914 known planet systems observed during TESS Cycle 3. This work has led to the new validation of 4 short-period planets. This study provides independent modeling and vetting of hundreds of planet candidates while also expanding the known planet population and providing updated transit ephemerides and planet radii.

Finally, I report on the atmospheric characterization of WASP-166b, a short-period super- Neptune (P = 5.44 d, Mp = 32.1 ± 1.6 M⊕, Rp = 7.1 ± 0.3 R⊕). WASP-166b is located near the edge of the Hot Neptune Desert, a sparse region of exoplanet parameter space at high stellar irradiation and intermediate planet radii. Using transmission spectroscopy of WASP-166b (two transit observations with the James Webb Space Telescope), initial analyses show evidence of H2O and CO2; no evidence of SO2, NH3, or a cloud deck; constraints on planetary metallicity and the C/O ratio; and a plausible formation pathway that includes planetesimal accretion followed by core erosion or photoevaporation. This in turn points to mechanisms that can create substellar or stellar C/O ratios and superstellar metallicities, like photoevaporation and core erosion, as feasible components of the formation of the Hot Neptune Desert.

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.

First, 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.

Next, 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.

Finally, 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.

To 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.