Discs of planetary-mass objects in σ Orionis

Aims. We searched for infrared flux excesses of planetary-mass candidates in the sigma Orionis cluster (similar to 3 Myr, similar to 350 pc). Methods. Using IJHK(s) data from the literature and the [ 3.6], [ 4.5], [ 5.8], and [ 8.0] IRAC images of the sigma Orionis cluster from the Spitzer Space Telescope public archives, we constructed colour-colour diagrams and spectral energy distributions from 0.8 to 8.0 mu m of cluster candidates fainter than J=18.0 mag, i.e. the planetary-mass borderline for sigma Orionis. Results. Infrared flux excesses are detected longward of 5 mu m in seven objects (SOri 54, 55, 56, 58, 60, SOri J053956.8-025315 and SOri J053858.6- 025228) with masses estimated in the range 7-14 M-Jup. Emission at shorter wavelengths ( 4.5 mu m) in excess of the photosphere is probably observed in SOri 56 and SOri J053858.6-025228. The faintest and least massive object, SOri 60, exhibits flux excess only at 8 mu m. We ascribe these infrared excesses to the presence of circumsubstellar warm discs, providing additional confirmation for the objects' membership of sigma Orionis. The observed incidence of inner discs around planetary-mass objects is >= 50%,which is consistent with the measured inner disc frequency among cluster brown dwarfs and low-mass stars, suggesting that these objects share a common origin. However, there is a trend for the inner disc rate to increase with decreasing mass ( from 10 M-circle dot through the substellar domain), which may be due to a mass-dependent timescale for the dissipation of the interior discs.