A population of very young brown dwarfs and free-floating planets in Orion

We describe the results of a very deep imaging survey of the Trapezium clus ter in the IJH bands, using the UKIRT high-resolution camera UFTI. Approxim ately 32 per cent of the 515 point sources detected are brown dwarf candida tes, including several free-floating objects with masses below the deuteriu m-burning (planetary) threshold at 0.013 M., which are detectable because o f their extreme youth. We have confidence that almost all the sources detec ted are cluster members, since foreground contamination is minimal in the 3 3-arcmin(2) area surveyed, and the dense backdrop of OMC-1 obscures all bac kground stars at these wavelengths. Extinction is calculated from the (J - H) colours, permitting accurate luminosity estimates, and temperatures are derived from the dereddened (I - J) colours. There is some evidence for a c ut-off in the luminosity function below the level corresponding to several Jupiter masses, which may represent the bottom end of the initial mass func tion. Since star formation is complete in the Trapezium, this limit could h ave wide significance, if confirmed. However, it could well be an effect of the dispersal of the molecular cloud by the central O-type stars, a proces s for which the time-scale will vary between star formation regions.