Low mass stellar and substellar population in young star forming regions: the Sigma Orionis case

The extension of the initial mass function towards the planetary mass regime and the search for the end of the mass function is extremely relevant to determine which is the dominant substellar formation process. I have been involved in the characterization of the lowest mass population (from ~13 down to 6 Jupiter masses) in the Sigma Orionis cluster (~3 Myr, ~380 pc). In a first stage, an homogenous study in the entire cluster area lead to the identification of 23 new planetary mass candidates plus a study of the spatial distribution of low–mass stars, brown dwarfs and planetary mass objects in the cluster. A second stage provides a mass function spectroscopically confirmed at the level of 75% from high-mass stars through the planetary–mass domain of the Sigma Orionis cluster. And finally, a study about the kinematics of the different mass regimes can relate their proper motion dispersions to the possible formation mechanisms discussed in the literature. I will summarize some of our most important results as well as the status of these kind of studies in other young star forming regions.