  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. We searched in the literature for stellar and
brown-dwarf member candidates of the sigma Orionis cluster 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. 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 Halpha 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 1700+/-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 ~45 M_Jup and an
L5+/-2 giant planet with an estimated mass of ~5 M_Jup. It would be the widest
and one of the lowest-mass planetary systems known so far.

Astrophysics