<P> Another possible mechanism for the formation of planetesimals is the streaming instability in which the drag felt by particles orbiting through gas creates a feedback effect causing the growth of local concentrations . These local concentration push back on the gas creating a region where the headwind felt by the particles is smaller . The concentration is thus able to orbit faster and undergoes less radial drift . Isolated particles join these concentrations as they are overtaken or as they drift inward causing it to grow in mass . Eventually these concentrations form massive filaments which fragment and undergo gravitational collapse forming planetesimals the size of the larger asteroids . </P> <P> Planetary formation can also be triggered by gravitational instability within the disk itself, which leads to its fragmentation into clumps . Some of them, if they are dense enough, will collapse, which can lead to rapid formation of gas giant planets and even brown dwarfs on the timescale of 1,000 years . If these clumps migrate inward as the collapse proceeds tidal forces from the star can result in a significant mass loss leaving behind a smaller body . However it is only possible in massive disks--more massive than 0.3 M. In comparison, typical disk masses are 0.01--0.03 M. Because the massive disks are rare, this mechanism of the planet formation is thought to be infrequent . On the other hand, this mechanism may play a major role in the formation of brown dwarfs . </P> <P> The ultimate dissipation of protoplanetary disks is triggered by a number of different mechanisms . The inner part of the disk is either accreted by the star or ejected by the bipolar jets, whereas the outer part can evaporate under the star's powerful UV radiation during the T Tauri stage or by nearby stars . The gas in the central part can either be accreted or ejected by the growing planets, while the small dust particles are ejected by the radiation pressure of the central star . What is finally left is either a planetary system, a remnant disk of dust without planets, or nothing, if planetesimals failed to form . </P> <P> Because planetesimals are so numerous, and spread throughout the protoplanetary disk, some survive the formation of a planetary system . Asteroids are understood to be left - over planetesimals, gradually grinding each other down into smaller and smaller bits, while comets are typically planetesimals from the farther reaches of a planetary system . Meteorites are samples of planetesimals that reach a planetary surface, and provide a great deal of information about the formation of the Solar System . Primitive - type meteorites are chunks of shattered low - mass planetesimals, where no thermal differentiation took place, while processed - type meteorites are chunks from shattered massive planetesimals . </P>

What is the biggest unsolved problem in the nebula disc model