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We present new observations of the planet beta Pictoris b from 2018 with GPI,the first GPI observations following conjunction. Based on these newmeasurements, we perform a joint orbit fit to the available relative astrometryfrom ground-based imaging, the Hipparcos Intermediate Astrometric Data (IAD),and the Gaia DR2 position, and demonstrate how to incorporate the IAD intodirect imaging orbit fits. We find a mass consistent with predictions ofhot-start evolutionary models and previous works following similar methods,though with larger uncertainties: 12.8 [+5.3, -3.2] M_Jup. Our eccentricitydetermination of 0.12 [+0.04, -0.03] disfavors circular orbits. We considerorbit fits to several different imaging datasets, and find generally similarposteriors on the mass for each combination of imaging data. Our analysisunderscores the importance of performing joint fits to the absolute andrelative astrometry simultaneously, given the strong covariance between orbitalelements. Time of conjunction is well constrained within 2.8 days of 2017September 13, with the star behind the planet's Hill sphere between 2017 April11 and 2018 February 16 (+/- 18 days). Following the recent radial velocitydetection of a second planet in the system, beta Pic c, we perform additionaltwo-planet fits combining relative astrometry, absolute astrometry, and stellarradial velocities. These joint fits find a significantly smaller mass for theimaged planet beta Pic b, of 8.0 +/- 2.6 M_Jup, in a somewhat more circularorbit. We expect future ground-based observations to further constrain thevisual orbit and mass of the planet in advance of the release of Gaia DR4.

The Gemini Planet Imager Exoplanet Survey: Dynamical Mass of the Exoplanet beta Pictoris b from Combined Direct Imaging and Astrometry