Astrometric Confirmation and Preliminary Orbital Parameters of the Young Exoplanet 51 Eridani b with the Gemini Planet Imager

We present new Gemini Planet Imager observations of the young exoplanet 51. Eridani. b that provide further evidence that the companion is physically associated with 51. Eridani. Combining this new astrometric measurement with those reported in the literature, we significantly reduce the posterior probability that 51. Eridani. b is an unbound foreground or background T-dwarf in a chance alignment with 51. Eridani to 2 x 10(-7), an order of magnitude lower than previously reported. If 51. Eridani. b is indeed a bound object, then we have detected orbital motion of the planet between the discovery epoch and the latest epoch. By implementing a computationally efficient Monte Carlo technique, preliminary constraints are placed on the orbital parameters of the system. The current set of astrometric measurements suggest an orbital semimajor axis of 14(-3)(+7) AU, corresponding to a period of 41(-12)(+35) years (assuming a mass of 1.75 M-circle dot for the central star), and an inclination of 138(-13)(+15) deg. The remaining orbital elements are only marginally constrained by the current measurements. These preliminary values suggest an orbit that does not share the same inclination as the orbit of the distant M-dwarf binary, GJ. 3305, which is a wide physically bound companion to 51. Eridani.