Physical and orbital properties of β Pictoris b

The intermediate mass star beta Pictoris is known to be surrounded by a structured edge on debris disk within which a gas giant planet as discovered orbiting at 8-10 AU. The physical properties of beta Pic b were previously inferred from broad- and narrow-band 0.9-4.8 mu m photometry. We used commissioning data of the Gemini Planet Imager (GPI) to obtain new astrornetry and a low-resolution (R similar to 35-39) J-band (1.12-1.35 mu 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(-0.6)(+0.4) 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 M-Jup (>= 96% prob.). The extracted spectrum of beta Pic h is similar to those of young L1(-1.5)(+1) dwarfs. We used the spectral type estimate to revise the planet luminosity to log(L/L-circle dot) = -3.90 +/- 0.07. The 0.9-4.8 mu m photometry and spectrum are reproduced for T-eff = 1650 +/- 150 K and a log g <= 4.7 dex by 2 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 beta Pic b are only reproduced by warm- and hot-start tracks with initial entropies S-i > 10.5 k(B)/baryon. These initial conditions may result from an inefficient 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 beta Pic b does not change these conclusions.