Direct confirmation of the radial-velocity planet beta Pictoris c

Context. Methods used to detect giant exoplanets can be broadly divided into two categories: indirect and direct. Indirect methods are more sensitive to planets with a small orbital period, whereas direct detection is more sensitive to planets orbiting at a large distance from their host star. This dichotomy makes it difficult to combine the two techniques on a single target at once.Aims. Simultaneous measurements made by direct and indirect techniques offer the possibility of determining the mass and luminosity of planets and a method of testing formation models. Here, we aim to show how long-baseline interferometric observations guided by radial-velocity can be used in such a way.Methods. We observed the recently-discovered giant planet beta Pictoris c with GRAVITY, mounted on the Very Large Telescope Interferometer.Results. This study constitutes the first direct confirmation of a planet discovered through radial velocity. We find that the planet has a temperature of T=125050 K and a dynamical mass of M=8.2 +/- 0.8 M-Jup. At 18.5 +/- 2.5 Myr, this puts beta Pic c close to a 'hot start' track, which is usually associated with formation via disk instability. Conversely, the planet orbits at a distance of 2.7 au, which is too close for disk instability to occur. The low apparent magnitude (M-K=14.3 +/- 0.1) favours a core accretion scenario.Conclusions. We suggest that this apparent contradiction is a sign of hot core accretion, for example, due to the mass of the planetary core or the existence of a high-temperature accretion shock during formation.