Four-of-a-kind: Characterising the HR 8799 system with GRAVITY

The four planets of HR8799 provide a unique opportunity to study the atmospheres of young, giant exoplanets with a shared formation history. In order explore these atmospheres, we use new data obtained with VLTI/GRAVITY as part of the ExoGRAVITY campaign, combined with data from SPHERE, GPI, CHARIS, ALES and OSIRIS in order to provide the best picture of the planetary atmospheres across a broad wavelength range. Using petitRADTRANS in a Bayesian retrieval framework, we compare a suite of state-of-the-art models applied to each of the targets in order to measure atmospheric properties such as metallicity, molecular abundances, and the C/O ratio. In this talk I will describe the data processing and modelling efforts which allow us to peer into the atmospheres of the HR8799 planets, and will outline the steps needed to tie the newly retrieved planetary properties to the formation history of the system