UC Santa Cruz

We have entered a new era of detailed exoplanet and brown dwarf atmospheric characterization with the James Webb Space Telescope (JWST), enabling very high signal-to-noise spectroscopic observations. This has allowed us to probe their atmospheres and interiors with unprecedented detail. This thesis introduces new constraints on previously uncertain processes such as atmospheric dynamics, cloud physics, and interior heat flux of transiting and self-luminous giant exoplanets, exo-Neptunes, and brown dwarfs, using newly developed advanced theoretical models and JWST observations.Constraints on Aerosols– Using 3D general circulation models, we examine whether the unknown 3D cloud distribution in exoplanets and brown dwarfs can be probed by their disk-integrated polarized thermal radiation. Our findings show that the clouds predicted in 3D GCMs can replicate the observed thermal polarization of brown dwarfs Luhman 16 A and B. However, we also show that small-scale vortices and cloud particle radii can sufficiently alter the disk-integrated polarized signals from such objects. To probe the 3D distribution pattern of aerosols in giant transiting exoplanet atmospheres, we present limb-resolved transmission spectra of the morning and evening limbs of hot Jupiter WASP-94A b, obtained using JWST. We find the colder morning limb of WASP-94A b is completely enshrouded in aerosols whereas the hotter evening limb is clear, showing prominent gas absorption features. We leverage these observations to show that aerosols in hot Jupiters can be primarily composed of condensate clouds instead of photochemical hazes. We also show that ignoring such aerosol-driven limb-to-limb differences can lead to severe (∼5σ) bias in their inferred composition, not only for hot Jupiters, but for the broader class of transiting exoplanets including smaller planets like sub-Neptunes.Constraints on Atmospheric Dynamics– Atmospheric dynamics is one of the least understood and most poorly constrained aspects of substellar atmospheres. Atmospheric dynamics along the radial direction, or vertical mixing, is often represented by the Kzz diffusion parameter. Theoretical and empirical uncertainty on Kzz spans several orders of magnitude. To constrain Kzz, we introduce PICASO 3.0, an opensource Python-based 1D radiative-convective equilibrium model that self-consistently treats disequilibrium chemistry from vertical mixing. Using PICASO 3.0, we demonstrate that JWST data can precisely constrain Kzz in both radiative and convective atmospheric regions. Our models predict brown dwarfs with higher gravities and temperatures of 500-900 K should have a second “detached” radiative region, which makes them ideal targets for constraining the very uncertain Kzz in their radiative atmospheres. To explore how metallicity, C/O ratio, and Kzz together shape the spectra of directly imaged planets and brown dwarfs, we present the Sonora Elf Owl model grid, which has been calculated with the PICASO model. We show that metallicity and Kzz driven vertical mixing can have degenerate effects on their spectra at various wavelength ranges. We use this model grid along with AKARI and Spitzer observations to show that free-floating brown dwarfs with temperatures between 500-900 K indeed have very low Kzz values, which agrees with our theoretical predictions of detached radiative regions in their deep atmospheres.Constraints on Interior and Atmospheric Properties– To assess whether JWST can constrain vertical mixing and interior properties of transiting planets, including warm-to-hot sub-Neptunes and gas giants, we integrate photochemistry into PICASO using the Photochem model. We show that CH4 is an ideal probe of Kzz in warm giants, while CO is better for warm sub-Neptunes. We also find that SO2 is no longer the main S- bearing gas in planets with Teq≤700 K or Teq ≥1100 K, with CS and CS2 becoming dominant. Using this model and JWST data, we constrain the atmospheric and interior properties of Neptune-class exoplanet GJ 436 b. We present the panchromatic JWST eclipse spectrum of GJ 436 b, showing it differs significantly from previously published Spitzer photometry in the 3.6 µm band. We find weak evidence of CO2 absorption in its spectrum. Using PICASO modeling, we find the planet’s day-side is either covered with a thick layer of aerosols with a metallicity ≥300×solar, or its interior is colder (Tint ∼ 60 K) with a metallicity ≥80×solar. Future observations of the planet, preferably in transmission, are required to distinguish between these scenarios and constrain its interior properties more precisely.