Spectroscopic estimate of surface gravity for a planetary member in the Sigma Ori cluster

We present intermediate-resolution (R=1500) near-infrared spectroscopy from 1.17 $μ$m to 1.37 $μ$m of the spectral type T planetary candidate member in the \sigori cluster S Ori 70 reported by Zapatero Osorio et al. (2002). 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$\pm$0.5 cm s$^{-2}$ and T$_{\rm eff}$=1,100$^{+200}_{-100}$ K. The low gravity of this object derived from spectral synthesis supports its youth and membership to the young \sigori cluster. Using evolutionary models for an age of 3 Myr, we obtain a mass of 3 \mjup and a radius of 0.16 $R_{\odot}$, 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.