The mass of the young planet Beta Pictoris b through the astrometric motion of its host star

The young massive Jupiters discovered with high-contrast imaging^1-4 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 models^5-8. 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 studies^9,10, 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 Hipparcos^13,14 and Gaia^15,16 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.