Statistical analyses of exoplanetary systems and individual studies of the atmospheres of two sub-Neptune-sized planets

Abstract

With over 4000 exoplanets already confirmed and more to come through various surveys, efforts have been made to measure their population properties and to characterize individual planets in terms of their mass, temperature, atmosphere, and evolution history. In this thesis, I present research on two exoplanet topics - planet occurrence rates and atmospheres of sub-Neptune-sized planets - through three major projects and one ongoing work. The first project is a uniform spectroscopy analysis of over 700 Kepler target stars, which disproves the popular hypothesis that the discrepancy between close-in gas giant occurrence rates measured from transit surveys and from the California radial velocity survey can be completely explained by their stellar metallicity difference, using the relation between occurrence rates and stellar properties.

The second project contains a uniform analysis on 64 Spitzer transit observations of 28 sub-Neptune-sized planets around M/K dwarfs, which enables the measurement of transmission spectral slopes of these planets from Kepler's broad optical bandpass to Spitzer's 4.5 pm IRAC channel. With these transmission spectral slopes, I propose that there exist two populations of cool small planets around lowmass stars characterized by their transmission spectral slopes, and that the smaller but significant population (20 ± 10% of all) could produce transmission spectra with detectable features. And in the third major project, a detailed measurement of the transmission spectrum of a sub-Neptune-sized planet HD 97658b is presented, using four HST/WFC3 observations, twelve Spitzer IRAC observations, and eight MOST observations. Subsequently, I discuss the implications of the transmission spectrum of HD 97658b on the atmospheric composition of this planet.

I also present the progress from ongoing research to measure the secondary eclipse of a super-Earth-sized planet TOI 561.02, discovered recently by TESS, around a solar-type star. The secondary eclipse could encode the day-side brightness temperature and composition of this planet. In addition to individual planet studies, I perform a uniform assessment on the transmission spectrum detectability of all currently confirmed small cool/warm planets with the upcoming JWST mission, and rank the best targets for future efforts.