Streaming Instability: How Pebbles Become Planetesimals

This is a local "shearing box" view of a patch of protoplanetary disk — a small window co-rotating with the gas, a few astronomical units from a young star. Millimeter-to-decimeter "pebbles" drift under gas drag according to the classic Weidenschilling radial-drift law, but wherever they happen to bunch up, their collective drag weakens the local headwind on the gas, which slows their drift there even further — a runaway feedback called the streaming instability, believed to be how real disks jump straight from pebbles to 10–100 km planetesimals without waiting on slow, collision-by-collision growth. Adjust the dust-to-gas ratio, pebble Stokes number and turbulence strength to see when clumping switches on, and watch overdense clumps collapse under self-gravity once they cross the Roche density — or turn that step off to see the filaments that form without it.