  (Abridged) The nature of S Ori 70, a faint mid-T type object found towards
the direction of the young sigma Orionis cluster, is still under debate. We
intend to disentangle whether it is a field brown dwarf or a 3-Myr old
planetary-mass member of the cluster. We report on near-infrared JHK_s and
mid-infrared [3.6] and [4.5] IRAC/Spitzer photometry recently obtained for S
Ori 70. The new near-infrared images (taken 3.82 yr after the discovery data)
have allowed us to derive a very small proper motion (11.0 +/- 5.9 mas/yr) for
this object, which is consistent with the proper motion of the cluster within
1.5 sigma the astrometric uncertainty. The colors (H-K_s), (J-K_s) and
K_s-[3.6] appear discrepant when compared to T4-T7 dwarfs in the field. This
behavior could be ascribed to a low-gravity atmosphere or alternatively to an
atmosphere with a metallicity significantly different than solar. Taking into
account the small proper motion of S Ori 70 and its new near- and mid-infrared
colors, a low-gravity atmosphere remains as the most likely explanation to
account for our observations. This supports S Ori 70's membership in sigma
Orionis, with an estimated mass in the interval 2-7 Mjup, in agreement with our
previous derivation.
