CFBDSIR 2149-0403: young isolated planetary-mass object or high-metallicity low-mass brown dwarf?

Aims. We conducted a multi-wavelength, multi-instrument observational characterisation of the candidate free-floating planet CFBDSIR J214947.2 040308.9, a late T-dwarf with possible low-gravity features, in order to constrain its physical properties. Methods. We analysed nine hours of X-shooter spectroscopy with signal detectable from 0.8 to 2.3 mu m, as well as additional photometry in the mid-infrared using the Spitzer Space Telescope. Combined with a VLT/HAWK-I astrometric parallax, this enabled a full characterisation of the absolute flux from the visible to 5 mu m, encompassing more than 90% of the expected energy emitted by such a cool late T-type object. Our analysis of the spectrum also provided the radial velocity and therefore the determination of its full 3D kinematics. Results. While our new spectrum confirms the low gravity and/or high metallicity of CFBDSIR 2149, the parallax and kinematics safely rule out membership to any known young moving group, including AB Doradus. We use the equivalent width of the K i doublet at 1.25 mu m as a promising tool to discriminate the e ff ects of low-gravity from the e ff ects of high-metallicity on the emission spectra of cool atmospheres. In the case of CFBDSIR 2149, the observed K i doublet clearly favours the low-gravity solution. Conclusions. CFBDSIR 2149 is therefore a peculiar late-T dwarf that is probably a young, planetary-mass object (2-13 M-Jup, < 500 Myr) possibly similar to the exoplanet 51 Eri b, or perhaps a 2-40 M-Jup brown dwarf with super-solar metallicity.