2012A&A...548A..26D

Using the CFBDSIR wide field survey for brown dwarfs, we identified CFBDSIRJ214947.2-040308.9, a late T dwarf with an atypically red J-K_S colour. We obtained an X-Shooter spectra, with signal detectable from 0.8µm to 2.3µm, which confirmed a T7 spectral type with an enhanced K_s-band flux indicative of a potentially low-gravity, young object. The comparison of our near infrared spectrum with atmosphere models for solar metallicity shows that CFBDSIRJ214947.2-040308.9 is probably a 650-750K, logg=3.75-4.0 substellar object. Using evolution models, this translates into a planetary mass object with an age in the 20-200Myr range. An independent Bayesian analysis from proper motion measurements results in a 87% probability that this free-floating planet is a member of the 50-120Myr-old AB Doradus moving group, which strengthens the spectroscopic diagnosis of youth. By combining our atmospheric characterisation with the age and metallicity constraints arising from the probable membership to the AB Doradus moving group, we find that CFBDSIRJ214947.2-040308.9 is probably a 4-7 Jupiter mass, free-floating planet with an effective temperature of ∼700K and a logg of ∼4.0, typical of the late T-type exoplanets that are targeted by direct imaging. We stress that this object could be used as a benchmark for understanding the physics of the similar T-type exoplanets that will be discovered by the upcoming high-contrast imagers.