Context :

Aims : Using the CFBDSIR wide field survey for brown dwarfs , we identified CFBDSIRJ214947.2-040308.9 , a late T dwarf with atypically red J - K _ { S } colour .

Methods : We obtained an X-Shooter spectra , with signal detectable from 0.8 ~ { } \mu m to 2.3 ~ { } \mu m , which confirmed a T7 spectral type with an enhanced K _ { s } -band flux indicative of a potentially low-gravity , young , object .

Results : The comparison of our near infrared spectrum with atmosphere models , for solar metallicity , shows that CFBDSIRJ214947.2-040308.9 is probably a 650-750 K , log g=3.75-4.0 substellar object .
Using evolution models , this translates into a planetary mass object , with an age in the 20-200 Myr range .
An independent Bayesian analysis from proper motion measurements results in a 87 % probability that this free-floating planet is a member of the 50-120 Myr old AB Doradus moving group , which strengthens the spectroscopic youth diagnosis .

Conclusions : 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 masses free-floating planet with an effective temperature of \sim 700 K and a log g of \sim 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 .