Letter to the Editor Physical and orbital properties of Pictoris b

The intermediate-mass star Pictoris is known to be surrounded by a structured edge-on debris disk within which a gas giant planet was discovered orbiting at 8 10 AU. The physical properties of Pic b were previously inferred from broad- and narrow-band 0.9 4.8 m photometry. We used commissioning data of the Gemini Planet Imager (GPI) to obtain new astrometry and a lowresolution (R 35 39) J-band (1.12 1.35 m) spectrum of the planet. We find that the planet has passed the quadrature. We constrain its semi-major axis to 10 AU (90% prob.) with a peak at 8:9 +0:4 0:6 AU. The joint fit of the planet astrometry and the most recent radial velocity measurements of the star yields a planet dynamical mass lower than 20 MJup ( 96% prob.). The extracted spectrum of Pic b is similar to those of young L1 +1:5 dwarfs. We used the spectral type estimate to revise the planet luminosity to log(L=L ) = 3:90 0:07. The 0.9 4.8 m photometry and spectrum are reproduced for Te = 1650 150 K and a log g 4:7 dex by 12 grids of PHOENIX-based and LESIA atmospheric models. For the most recent system age estimate (21 4 Myr), the bolometric luminosity and the constraints on the dynamical mass of Pic b are only reproduced by warm- and hot-start tracks with initial entropies S i > 10:5 kB=baryon. These initial conditions may result from an ine cient accretion shock and/or a planetesimal density at formation higher than in the classical core-accretion model. Considering a younger age for the system or a conservative formation time for Pic b does not change these conclusions.