Jerry Xuan (UCLA): "A Golden Age for Gas Giant Atmospheres with JWST"

With over six thousand discovered exoplanets, we are firmly in the age of exoplanet characterization. In this talk, I will connect exquisite observations of exoplanet atmospheres to the fundamental question of how gas giants form and evolve. In particular, the JWST/NIRSpec and MIRI integral field units are delivering some of the highest-quality spectra of directly imaged gas giants, revealing rich molecular features from CO, CH4, H2O, H2S, CO2, NH3, as well as isotopologues. First, I will discuss recent results for the HR 8799 system, in which compositional trends across four gas giants suggest the accretion of both solids and metal-enriched gas. I place these findings in the broader context of chemical abundances for directly imaged companions, and highlight an emerging compositional divide where objects like HR 8799 bcde and AF Lep b are enriched in multiple heavy elements, whereas widely-separated substellar companions with mass ratios ≳ 1% share the same chemical compositions as their host stars. Finally, I showcase comprehensive spectroscopy and photometry (3-25 microns) for eps Indi Ab, the first mature (~4 Gyr) cold Jupiter (Teff~275 K) imaged around another star.