Context : Free-floating planetary-mass objects have masses below the deuterium burning mass limit at about 13 Jupiter masses , and have mostly been found in very young open clusters .
Their origin and relationship to stars and brown dwarfs are still a mystery .

Aims : The recent detection by direct imaging of three giant planets at wide separation ( 50–250 AU ) from their primaries has raised the question about the “ true isolation ” of planetary-mass objects in clusters .
Our goal was to test the possibility that some free-floating planetary-mass object could in fact be part of wide planetary systems .

Methods : We searched in the literature for stellar and brown-dwarf member candidates of the \sigma Orionis cluster ( \sim 3 Ma , \sim 360 pc ) at small angular separations from published candidate planetary-mass objects .
We found one candidate planetary system , SE 70 , composed of an X-ray source and a planetary-mass object , namely S Ori 68 , separated by only 4.6 arcsec .
In order to assess the cluster membership of the X-ray source , we obtained mid-resolution optical spectroscopy using ISIS on the William Herschel Telescope .
We also compiled additional data on the target from available astronomical catalogues .

Results : We have found that SE 70 follows the spectrophotometric sequence of the cluster and displays spectroscopic features of youth , such as lithium in absorption and chromospheric H \alpha emission .
The radial velocity is consistent with cluster membership .
Hence , SE 70 is very probably a \sigma Orionis cluster member .
The projected physical separation between SE 70 and S Ori 68 is 1 700 \pm 300 AU at the distance of the cluster .
If the common proper motion is confirmed in the near future , the system would be composed of an M5–6 brown dwarf with an estimated mass of \sim 45 M _ { Jup } and an L5 \pm 2 giant planet with an estimated mass of \sim 5 M _ { Jup } .
It would be the widest and one of the lowest-mass planetary systems known so far .

Conclusions :