The young massive Jupiters discovered with high-contrast imaging1,2,3,4 provide a unique opportunity to study the formation and early evolution of gas giant planets. A key question is to what extent gravitational energy from accreted gas contributes to the internal energy of a newly formed planet. This has led to a range of formation scenarios from ‘cold’ to ‘hot’ start models5,6,7,8. For a planet of a given mass, these initial conditions govern its subsequent evolution in luminosity and radius. Except for upper limits from radial velocity studies9,10, disk modelling11 and dynamical instability arguments12, no mass measurements of young planets are yet available to distinguish between these different models. Here, we report on the detection of the astrometric motion of Beta Pictoris, the ~21-Myr-old host star of an archetypical directly imaged gas giant planet, around the system’s centre of mass. Subtracting the highly accurate Hipparcos13,14 and Gaia15,16 proper motion from the internal 3 yr Hipparcos astrometric data reveals the reflex motion of the star, giving a model-independent planet mass of 11 ± 2 Jupiter masses. This is consistent with scenarios in which the planet is formed in a high-entropy state as assumed by hot start models. The ongoing data collection by Gaia will soon lead to mass measurements of other young gas giants and form a great asset to further constrain early-evolution scenarios.
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This work was only possible because numerous scientists and engineers have devoted large parts of their careers to the design, construction and successful operation of the Hipparcos and Gaia missions and the analysis of their data. We are grateful. We thank F. van Leeuwen also for discussions on his analysis of the Hipparcos data. This work has made use of data from the European Space Agency mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. I.A.G.S. acknowledges funding from the European Research Council under the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 694513. A.G.A.B. acknowledges funding from the Netherlands Research School for Astronomy (NOVA).