Direct imaging is the only exoplanet detection method that gives no model-independent information about planet mass or radius. Such fundamental parameters are not only important for characterizing the population of gas giants in wide orbits, but they are also critical for the testing of formation and evolutionary models. One of the most exciting developments in the field of direct imaging since the last Protostars and Planets meeting has been the advent of dynamical mass measurements for planets derived from ultra-precise astrometry of their host stars. I will present our work using the Hipparcos-Gaia Catalog of Accelerations to measure the masses of planets in the beta Pictoris, HR 8799, and 51 Eri systems. The dynamical masses are broadly consistent with expectations from "hot-start" evolutionary models, and in the case of 51 Eri b our mass allows us to place the first constraints on its initial entropy. From Keck adaptive optics orbit monitoring, we have measured precise orbital parameters and dynamical mass for the host binary in the VHS J1256 system. This enables the first precise age dating of its companion, which implies a mass on the borderline of the deuterium-fusion boundary. The highly eccentric orbit that we find for VHS J1256 AB suggests a dynamical origin for its wide companion. The future promises many more dynamical mass measurements for giant planets, both from the increased precision of further Gaia data releases, and also the discovery of new planets that display accelerations like AF Lep b.
[Poster PDF File]