PSO J318.5338 - 22.8603 is an extremely-red planetary-mass object that has been identified as a candidate member of the \beta Pictoris moving group based on its spatial position and tangential velocity .
We present a high resolution K -band spectrum of PSO J318.5338 - 22.8603 .
Using a forward-modeling Markov Chain Monte Carlo approach , we report the first measurement of the radial velocity and v sin ( i ) of PSO J318.5 - 22 , - 6.0 ^ { +0.8 } _ { -1.1 } km s ^ { -1 } and 17.5 ^ { +2.3 } _ { -2.8 } km s ^ { -1 } , respectively .
We calculate the space velocity and position of PSO J318.5 - 22 and confirm that it is a member of the \beta Pictoris moving group .
Adopting an age of 23 \pm 3 Myr for PSO J318.5 - 22 , we determine a mass of 8.3 \pm 0.5 M _ { Jup } and effective temperature of 1127 ^ { +24 } _ { -26 } K using evolutionary models .
PSO J318.5338 - 22.8603 is intermediate in mass and temperature to the directly-imaged planets \beta Pictoris b and 51 Eridani b , making it an important benchmark object in the sequence of planetary-mass members of the \beta Pictoris moving group .
Combining our v sin ( i ) measurement with recent photometric variability data , we constrain the inclination of PSO J318.5 - 22 to > 29 ^ { \circ } and its rotational period to 5–10.2 hours .
The equatorial velocity of PSO J318.5 - 22 indicates that its rotation is consistent with an extrapolation of the velocity-mass relationship for solar system planets .