2003ApJ...593L.113M

We present intermediate-resolution (R=1500) near-infrared spectroscopy from 1.17 to 1.37 µm of the spectral type T planetary candidate member in the σ 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 logg=3.5±0.5 cm.s^–2 and T_eff=1100⁺²⁰⁰_–100K. The low gravity of this object derived from spectral synthesis supports its youth and membership to the young σ Orionis cluster. Using evolutionary models for an age of 3 Myr, we obtain a mass of 3 M_Jupand 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.