Characterizing the Atmospheres of Exoplanet Populations: From Sub-Jovian to Ultra-hot Jupiter Exoplanets

Abstract

With thousands of exoplanets now discovered and the diversity of worlds beyond our solar system being unveiled, we are now beginning to understand exoplanet populations in detail. The most direct way that we can characterize an exoplanet is by studying its atmosphere. This dissertation uses both observations and modeling to explore the properties of different exoplanet populations. Using transit spectroscopy with HST/STIS, I observed the atmosphere of the JWST GTO target GJ 436b. The resulting optical spectrum is consistent with a cloudy, moderate metallicity atmosphere. I find intriguing similarities in the optical transit spectrum of several sub-Jovian exoplanets and explore possible explanations. While sub-Jovian exoplanets are on the frontier of characterization, ultra-hot Jupiters are some of the most ideal targets for observation due to their high temperature, inflated radii, and short periods. Using the self-consistent PHOENIX atmosphere model, I show that the atmospheres of ultra-hot Jupiters exhibit some unique properties, including thermal dissociation and intense temperature inversions, even in the absence of TiO or VO. I also show that my models qualitatively match the characteristics seen in many ultra-hot Jupiter observations. Since ultra-hot Jupiters are amongst the most highly irradiated objects, I explore the role that the host star irradiation spectrum plays in the planet’s temperature structure and composition, finding that ultra-hot Jupiters around hotter host stars will have more intense thermal inversions. I also further quantify the opacity sources responsible for heating ultra-hot Jupiter atmospheres. Lastly, I present some novel techniques to characterize exoplanet atmospheres using PETRA, a new retrieval framework I have built around PHOENIX.