Characterizing the Atmospheres of Extrasolar Worlds

More than 5,000 extrasolar planets have been discovered to date, and we now know that planets are ubiquitous. The most commonly used techniques used to discover these distant worlds simultaneously measure their orbital periods, as well as their radii and/or masses. In the many cases where both the radii and masses have been measured it is possible to estimate the bulk planetary composition, which has yielded great insight into the incredible diversity planets exhibit. To venture beyond the conclusions that can be drawn from orbital configurations, masses, and radii, the natural next step in advancing our understanding of planets has been to characterize their atmospheres. They hold information on the planetary chemical composition, temperature structures, and atmospheric dynamics, and provide insight into their formation, evolution, and current climate. In this thesis, I use the transmission spectroscopy technique and present atmospheric observations obtained with the Hubble Space Telescope for the two hot Jupiters WASP-79b and WASP-101b. For WASP- 79b, I report an optical transmission spectrum that displays an unusual slope with decreasing transit depths towards short wavelengths. This is in contrast to expectations from Rayleigh scattering or hazes and I instead find that the spectrum is best described by including unocculted faculae on the stellar surface as well as H^- and H₂O in the planetary atmosphere. For WASP-101b, I find the flattest transmission spectrum observed to date. This finding is consistent with the presence of a high-altitude cloud deck in the atmosphere, which I speculate is likely dominated by silicates, blocking the view into deeper parts of the atmosphere. Potential future observations with the James Webb Space Telescope (JWST) offer the intriguing possibility to directly measure ionic species in the atmosphere of WASP-79b and to test the hypothesis that the clouds inWASP-101b are composed of silicates. Furthermore, I am leading a program with JWST to probe the Earth-sized planet, TRAPPIST-1c, with the possibility to detect the presence of a compact secondary atmosphere for the first time.