Abstract

As the poster child of directly-imaged exoplanets, the HR 8799 system consists of four young gas giant planets that are likely to be in mean motion resonance. Their formation origin and evolutionary history remain uncertain but can be constrained by a rich set of archival and newly-obtained proprietary data. I will present the detection of key molecular species such as H2O and CO in the planetary atmosphere using the combination of high-contrast imaging and high-resolution spectroscopy. Despite solid detections, interpreting the data through spectral retrieval analysis has been controversial, largely due to systematics introduced by inhomogeneous data sets and differing underlying model assumptions. I will present a calibration procedure using benchmark brown dwarfs to understand the systematic errors. This leads to an accurate constraint of atmospheric composition, which in turn allows us to robustly trace the formation history. Moreover, using a sample of directly-imaged exoplanets including the HR 8799 planets, we convert their atmospheric metallicities to the accreted mass of solid during formation through a Bayesian framework that marginalizes the probabilities of disk conditions, formation locations, planetary interior structures, and accretion physics. We show that more than 50 M⊕ solid mass has been accreted, pointing to an extremely early formation time scale (