Post conjunction detection of $\beta$ Pictoris b with VLT/SPHERE

With an orbitaldistance comparable to that of Saturnin the solar system, \bpic b is the closest ( semi-major axis$\simeq$\,9\,au) exoplanetthat has been imaged to orbita star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope(VLT), all following astrometric measurements relative to \bpic have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet's orbitalparameters. We aimed at further constraining \bpic b orbitalproperties using more data, and, in particular, data taken in the northeastern part of the orbit. We used SPHERE at the VLT to precisely monitor the orbital motionof beta \bpic b since first lightof the instrument in 2014. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6\,au) and prevented further detection. We redetected \bpic b on the northeast side of the disk at a separation of 139\,mas and a PA of 30$^{\circ}$ in September 2018. The planetary orbitis now well constrained. With a semi-major axis(sma) of $a = 9.0 \pm 0.5$ au (1 $\sigma $), it definitely excludes previously reported possible long orbital periods, and excludes \bpic b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbitat less than 2 au from a star that is 20 pc away.