Brown dwarfs are objects that bridge the realms of stars and planets. With masses below ~0.08 MSun, they cannot sustain hydrogen fusion, never reach the main sequence, and remain cooling forever. Star forming regions and young clusters harbour large populations of these substellar objects, including some with masses comparable to those of giant extrasolar planets. Understanding how brown dwarfs form is one of the crucial missing pieces in our understanding of how star and planet formation work. Deep surveys in young clusters are of particular importance, as they help to determine the shape and the low-mass limit of the stellar mass distribution at birth, which are fundamental constraints on star formation theory.
In this talk I will present the main results of our deep survey SONYC (Substellar Objects in Nearby Young Clusters), designed to provide a census of the substellar population in nearby star forming regions, and characterize their Initial Mass Functions down to unprecedented masses below 10 MJup. Furthermore, I will present our new program to study brown dwarf populations in a sample of massive young clusters, characterized by high stellar densities and presence of many massive OB stars. Characterizing substellar populations in star forming environments that are drastically different from those where brown dwarfs have been studied so far, allows us for the first time to look for variations in the efficiency of brown dwarf formation in the Milky Way as a function of environment.