We present new Gemini Planet Imager observations of the young exoplanet 51 Eridani b which 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 \times 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 ^ { +7 } _ { -3 } AU , corresponding to a period of 41 ^ { +35 } _ { -12 } years ( assuming a mass of 1.75 M _ { \odot } for the central star ) , and an inclination of 138 ^ { +15 } _ { -13 } deg .
The remaining orbital elements are only marginally constrained by the current measurements .
These preliminary values suggest an orbit which 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 .