In-situ formation by gravitational instability
It is quite likely that during the earliest stages of star formation, protostellar disc will be massive relative to the mass of the central star and that their evolution will be driven primarily by self-gravity. If such discs become extremely gravitationally unstable, they may fragment to form bound objects that could contract to form giant planets or sub-stellar objects. It is now generally accepted, however, that such a process cannot operate effectively in the inner parts of these discs, but that it may operate at large radii (r > 50 AU). We consider here the conditions required for discs to undergo fragmentation and discuss when this should happen, what types of objects might form, and if there is any current observational evidence to support such a process.