streaming instability

Streaming instability is a theoretical mechanism presumed to disrupt protoplanetary disks such that solid particles do not remain spread out, but gather, giving them a better chance to clump and form planetary embryos, a theoretical mechanism conceived in the modeling of planet formation. The gas's pressure provides an outward force above and beyond centrifugal force, thus the gas orbits at less than Keplerian speed. Solid particles sufficiently massive to reduce their coupling to the gas orbit closer to Keplerian speed making them experience a headwind, which affects them over time, slowing them, resulting in an inward radial drift (as per the radial-drift barrier). This effect is somewhat reduced by the fact that the particles drag surrounding gas along a bit, reducing the headwind. This reduction is increased for groups of solid particles so groups drift more slowly than single particles. So instead of particles throughout the disk drifting inward "in parallel", any particles just outside such a group drifts into the group's orbit, enlarging the group, which in turn increases the effect, i.e., positive feedback, so a disk with solid particles (initially) roughly evenly spread is unstable. This increases the probability of particles clumping within the disk, increasing the probability of the formation of planetary embryos.