Spectroscopic estimate of surface gravity for a planetary member in the σ Orionis cluster

We present intermediate-resolution (R = 1500) near-infrared spectroscopy from 1.17 to 1.37 mum of the spectral type T planetary candidate member in the sigma Orionis cluster S Ori 70 reported by Zapatero Osorio et al. The new data have been obtained with NIRSPEC at the Keck II telescope. The best fit of our mid-resolution spectrum of S Ori 70 with theoretical spectra gives log g = 3.5 +/- 0.5 cm s(-2) and T-eff = 1100(-100)(+200) K. The low gravity of this object derived from spectral synthesis supports its youth and membership to the young sigma Orionis cluster. Using evolutionary models for an age of 3 Myr, we obtain a mass of 3 M-Jup and a radius of 0.16 R., independent of the distance to the object. Our analysis confirms that S Ori 70 is the lowest mass cluster planet so far identified in the galaxy.