[Note: this proposal was accepted and (near-)top-ranked in S21B/S22A but not fully executed due to observatory failures and weather.] We propose SCExAO/CHARIS follow-up characterization of a faint, benchmark directly-imaged exoplanet: HIP 99770 b (Currie et al. 2022, Science under final editorial revisions, to be published) orbiting ∼ 17 au from its host star. Analysis of its spectrum favors an atmosphere comparable in temperature but intermediate in cloudiness between field brown dwarfs and the first imaged exoplanets, HR 8799 bcde. It is responsible for a 3σ acceleration induced on its host star as seen by Gaia: unlike nearly all other imaged exoplanets so far, it is possible to derive the companion’s dynamical mass instead of inferring a mass from uncertain evolutionary models. New data will double our available astrometric baseline and yield a high SNR detection across the JHK passbands that better constrain’s the companion’s atmosphere, orbit, and dynamical mass. HIP 99770 b is a proof-of-concept for a fundamentally different type of exoplanet direct imaging search, using ultra-precise Gaia astrometry to identify stars with imageable planets.
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