The First Million Years The growth of planet-sized bodies in a protoplanetary disk ("proplyd") is thought to occur essentially through collisions. Earlier models relied on gravitational instabilities in the dust layer to rapidly grow objects with sizes of several kilometres. However, it has been pointed out that the differences between the dust and gas velocities will stir up the dust sufficiently to make such instabilities impossible. While the extent of this turbulence in the dust layer is still a subject of debate, collisional growth from the smallest sizes and on to larger bodies has now become the scenario favoured by scientists. From microscopic grains to metre-sized bodies At first, dust grains collide at relatively gentle velocities which are determined by the "gas drag", that is, the friction between the moving dust grains and the surrounding gas. This force is dependent on the sizes and shapes of the dust grains. As the small bodies grow larger, the importance of the "gas drag" diminishes and it disappears completely by the time the objects reach sizes of the order of several tens of metres. At this stage, we start to talk about "planetesimals". From metre to kilometre-sized bodies As the mass of these bodies increases further, their gravitation begins to play a role and more and more collisions take place. Soon, the so-called "runaway growth phase" is reached during which the larger bodies sweep-up all those smaller ones that happen to come within their gravitational reach. This phase is not without theoretical problems, though. Laboratory experiments have shown that very small dust grains aggregate readily. On the very large scales, various impact simulations have shown that self-gravity will ensure growth. Sticky problems However, the situation is much less clear for objects ranging in size from a centimetre to some kilometres. In this size range, no real "sticking" mechanism has yet been found. Indeed, at this size, the forces operating at the micron size level are no longer effective and gravity is still much too weak. The escape velocity of a 1 metre-sized rock is of the order of 1 mm/s, while the typical collisional velocities between these objects are of the order of 100 m/s. Hence, for sticking the involved bodies together, they must somehow "get rid of" all but 10-10 of their kinetic (motion) energy. It still remains to be seen whether this can be achieved by purely mechanical structures or perhaps requires the presence of a kind of "glue" with special properties.

Life in the Universe
  Formation of Planetary Systems
    Planetary Formation
      Protoplanetary Disks (To be added soon!)
      The First Million Years
      The Next 100 Million Years
      Formation of Giant Planets
      Planet Migration