Cradle to climate: tracing the composition and dynamics of extrasolar atmospheres

Abstract

Over the past decades, a growing number of giant exoplanets have been directly imaged next to their host stars. Understanding their atmospheres in detail and how these planets even came to be, however, remains challenging. Free-floating brown dwarfs, which cool through the same range of temperatures, serve as valuable analogues to better probe such extrasolar atmospheres. This thesis uses high-resolution spectroscopy from the ground and space-based observations with JWST to characterise the atmospheres of an isolated brown dwarf, the nearest brown-dwarf binary, and a planetary-mass companion. Atmospheric retrieval models are used to measure the elemental and isotopic composition which helps to inform the object's formation environment. All targets are affected by dynamical processes, exhibiting signs that vertical mixing and patchy cloud covers shape their chemistry and spectral appearance. In addition, I introduce a new tool for computing the molecular and atomic opacities needed for this kind of analysis. Together, these studies demonstrate how chemical compositions can be used to trace the origin and atmospheric dynamics of both brown dwarfs and giant exoplanets.