Dr. Zhoujian Zhang, Postdoctoral Sagan Fellow, University of California, Santa Cruz
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Abstract: Over the past two decades, the broad outline of exoplanet demographics has been sketched. The next decade promises to be defined by our ability to explain the formation and evolution of these planets through the lens of their compositions, masses, and birth environments. There is a community-wide interest in studying planet formation by measuring their elemental abundances (e.g., carbon-to-oxygen ratio, bulk metallicity, isotopologues), given that different formation processes yield distinct relative compositions between planets and their parent stars. In this talk, I will introduce an ongoing spectroscopic program designed to measure the elemental abundances across an extensive array of directly imaged exoplanets, benchmark brown dwarfs, and their parent stars. I will spotlight a remarkable planetary system, AF Lep A+b, to illustrate our analysis approaches. Our spectroscopic analysis of planets and brown dwarfs leverages (1) a novel retrieval framework that we developed to robustly characterize cloudy self-luminous atmospheres, and (2) a Bayesian forward-modeling framework enhanced by machine learning techniques and Gaussian processes. By applying these methods to the benchmark brown dwarfs within our sample, we have been quantifying the systematic errors inherent in state-of-the-art model atmospheres, enabling reliable atmospheric characterization for gas-giant planets. Looking ahead, the wealth of forthcoming data from large sky surveys such as LSST and Gaia DR4 will substantially expand the targets of our program, enabling a population-level understanding of planet formation processes.
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