What condition triggers the streaming instability pathway for rapid planetesimal formation?
Answer
Solid material fraction reaching around 1% or more of the disk mass.
The streaming instability is a mechanism hypothesized to rapidly overcome the meter barrier by accelerating growth past the drag-dominated phase. This process is initiated when the concentration of solid material—pebbles—within the gaseous disk becomes sufficiently high, generally cited as around 1% or greater of the total disk composition. At this critical density threshold, the collective gravitational influence of the solids overcomes the retarding effect of gas drag. This allows the entire swarm of pebbles to collapse rapidly under its own self-gravity, forming kilometer-sized planetesimals in just a few orbital periods, thereby transforming the disk structure quickly.
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