Exoplanet Demographics Beyond Kepler: Giant Planets with Radial Velocity & Young Transiting Planets with TESS

Abstract

Large scale exoplanet detection and characterization surveys have made it possible for us to study exoplanets from a demographic perspective. Of specific importance is the \kepler mission, which has provided detailed exoplanet population statistics for a large range of transiting planet sizes orbiting close to their host stars. In this dissertation, I carefully compare the intrinsic occurrence of (i) giant planets across various detection techniques (transit vs. radial velocity vs. direct imaging), and (ii) young vs. mature Neptune-sized planets, in order to better understand the physical mechanisms that drive planet formation, evolution, and migration. In the first half of my dissertation, I focus on the orbital distribution of giant planets which plays an important role in the formation of terrestrial planets and their habitability. I compute the intrinsic giant planet occurrence rate as a function of orbital period by taking into account the detection efficiency of the Kepler survey as well as the Mayor et al. (2011) HARPS/CORALIE radial velocity survey. I find that there is a peak in the radial velocity giant planet occurrence rate distribution that coincides with the location of the snowline of Sun-like stars ~2.5 au. An extrapolation out to larger semi-major axes (~100 au) matches the 1% planet occurrence rates from direct imaging surveys. Furthermore, various planet population synthesis models show good agreement with the observed distribution, and can be used to constrain giant planet formation and migration mechanisms. In the second half of my dissertation, I delve into understanding the evolution of Kepler's short-period sub-Neptunes and Neptunes over time. The Gyr-old population of short-period planets has some distinctive features such as the radius valley and the hot Neptune desert, both of which indicate that this population is likely sculpted by atmospheric mass loss, a process that can even lead to these planets losing their envelopes entirely, leaving behind just their bare cores. To explore this further, I use the Transiting Exoplanet Survey Satellite (TESS) mission to detect and characterize short-period Neptunes around stars in young clusters (<1 Gyr). These planets represent a sample much closer in time to the primordial planet population, before atmospheric mass loss has significantly eroded their atmospheres. My preliminary estimates of the intrinsic occurrence of young short-period Neptunes indicate that the frequency of these planets has decreased over time. This suggests that atmospheric mass loss plays an important role in the atmospheric and radial evolution of the short-period planet population.