The past decade has seen the discovery of more than 130 giant extrasolar
planets. The masses of these planets range from as small as that of
Neptune to ten times bigger than the mass of Jupiter and their orbits show
all sorts of eccentricties and mean radii. While NASA and ESA are
delivering large ground-based and space-based observational programs to
learn more about these giant planets and eventually search for
terrestrial planets, these exciting discoveries are
producing a revision of models of planet formation that were once suited
only to explain the Solar System.  One of such new theories, disk
instability, will be discussed today. I will describe state-of-the art 3D
hydrodynamical (SPH) simulations that study the evolution of
gaseous protoplanetary disks formed by the high angular momentum material
during the process of star formation. I will show that these disks
can fragment rapidly into clumps following the onset
of gravitational instability. These clumps are gravitationally
bound,long-lasting and have masses and orbits comparable to those of
extrasolar giant planets. I will then discuss how the balance of heating
and cooling in the disk is crucial for the onset of fragmentation and will
also show new calculations which incorporate flux-limited radiation
transport.