Dust coagulation

Dust coagulation is the sticking and growth of dust grains through collisions. It occurs in environments such as protoplanetary disks, where it contributes to planet formation.

Brownian motion: Random motion of particles suspended in a fluid, driven by collisions with the fluid’s molecules. It brings the smallest dust grains into collisions before larger grains dominate.
Protoplanetary disk: A disk of gas and dust surrounding a young star, where planets can form. It provides the setting in which dust coagulation can begin building planetary solids.
Dust grain: A small solid particle suspended in a gas or embedded in a larger body. Individual grains are the colliding units whose aggregation defines dust coagulation.
Meter-size barrier: The difficulty of growing solid bodies through sizes where rapid inward drift and destructive collisions become severe. It captures why coagulation may stall before aggregates reach planetesimal scales.
Sticking probability: The likelihood that colliding particles remain attached rather than separating. It sets how efficiently dust collisions turn into growth.
Planetesimal: A solid body formed from smaller bodies that can participate in building a planet. Coagulation is one proposed route from fine dust toward the precursors of planetesimals.
Gas drag: The force exerted by gas on a solid particle moving relative to it. Gas drag shapes particle motion and relative collision speeds in disks.
Radial drift barrier: The loss of solid particles toward a star as gas drag removes their orbital energy and angular momentum. Growing particles can drift inward faster than coagulation can increase their size.
Coefficient of restitution: A measure of how much relative speed two colliding bodies retain after impact. It helps distinguish bouncing collisions from dissipative encounters that can lead to sticking.
Streaming instability: A collective instability in a gas–solid mixture that can concentrate particles and promote gravitational collapse. It may help aggregates overcome the growth bottlenecks left by collisions alone.

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