The Origins and Properties of Giant Planets on Extreme Orbits

Over the past three decades we have discovered thousands of exoplanets orbiting nearby stars, greatly expanding our understanding of the galactic planet population. Most of these exoplanetary systems have properties that are markedly different from those of our own solar system. Some systems contain gas giant planets with masses comparable to or even larger than that of Jupiter orbiting only a few stellar radii from the surfaces of their host stars. In other systems, gas giants have been discovered with orbital separations more than 100 times the Earth-Sun distance. Both of these populations challenge our understanding of gas giant planet formation, and it has been suggested that they might have initially formed at intermediate orbital separations and then migrated to their present-day locations. In this thesis, I combine multiple observational techniques to characterize the properties of giant planets on extreme orbits in order to constrain their formation and migration histories. In Chapter 1, I fit both low- and high-resolution infrared spectra of the massive, widely separated, planetary-mass companion ROXs 42B b in order to measure the elemental abundances of carbon and oxygen in its atmosphere. In Chapter 2, I use two decades of astrometric monitoring to measure the mutual inclination between the orbital plane of ROXs 42B b and that of its host binary. In Chapter 3, I demonstrate how mid-infrared emission spectroscopy with JWST/MIRI LRS can be used to characterize the properties of both clouds and gas-phase absorbers in the atmosphere of HD 189733 b, a hot gas giant on a close-in orbit. In Chapter 4, I present a preliminary analysis of new high-resolution optical spectroscopy of the second hottest transiting planet currently known, TOI-2109 b. I use these optical high-resolution spectra to detect the presence of Fe, Mg, and Ti in this planet's atmosphere, and discuss how the relative abundances of refractory and volatile species can be used to constrain the solid-to-gas ratio of materials incorporated into the planet's envelope. In the conclusion, I discuss how the complementary techniques utilized in this thesis can be combined to develop a holistic view of the properties of gas giant planets on extreme orbits, which in turn allows us to test competing models for their origin.