Discs around young free-floating planetary-mass objects: ultradeep Spitzer imaging of IC348

Protoplanetary discs have been found around free-floating objects with masses comparable to those of giant planets. The frequency and properties of these discs around planetary-mass objects are still debated. Here, we present ultradeep mid-infrared images for the young cluster IC348, obtained through stacking of time-series images from Spitzer. We measure fluxes at 3.6 and 4.5 mu m for known free-floating planetary-mass objects (FFPMOs, spectral type M9 or later) in this cluster. By comparing the observed infrared spectral energy distributions with photospheric templates, we identify six planetary-mass objects with discs, plus three, which may or may not have a disc. This corresponds to a disc fraction of 46 +/-(13)(12 )per cent. The disc fraction among planetary-mass objects is comparable to more massive brown dwarfs. We show the disc fraction among FFPMOs as a function of age, demonstrating that these objects retain discs for several million years, similar to low-mass stars and brown dwarfs.