Planetesimal formation
Planets form in protoplanetary discs around young stars as dust grains collide and stick together to form larger and larger bodies. The formation of km-sized planetesimals from smaller cm-dm sized particles nevertheless faces major difficulties in the traditional coagulation scenario. Such particles do not stick well and very quickly drift towards the star to sublimate in the inner nebula.
Planetesimal formation takes place in a complex environment of turbulent gas interacting via drag forces with particles of many sizes. The streaming instability thrives in the systematic relative motion of gas and particles and leads to spontaneous clumping of particles. The gravity from the star is partially compensated by the radial pressure gradient of the gas, causing gas to orbit at a slightly slower speed than Keplerian. Clumping is initiated as initially very low amplitude particle overdensities accelerate the gas towards the Keplerian speed, hence reducing the local head-wind on the particles, which in turn slows the radial drift of the particles. Drifting particles pile up where the head-wind is slower, causing exponential growth of the particle density as the particles continue to increase their drag force influence on the gas.
The process can be compared to how bicycle riders and migrating geese travel together to protect themselves from the headwind of the atmosphere.
IMAGE 1:
The figure shows the spontaneous transition from laminar flow to a turbulent state where dense particle filaments continuously form and break up.
The particle filaments become very dense, up to several thousand times the local gas density. The collective gravity of all the particles is high enough to initiate a gravitational collapse to form planetesimals with a characteristic radius of 500 km. This is comparable to the dwarf planet Ceres in the asteroid belt.
IMAGE 2:
The figure shows the formation of planetesimal by gravitational collapse of pebbles and rocks that have first been concentrad by the streaming instability.
The particle filaments become very dense, up to several thousand times the local gas density. The collective gravity of all the particles is high enough to initiate a gravitational collapse to form planetesimals with a characteristic radius of 500 km. This is comparable to the dwarf planet Ceres in the asteroid belt.