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Title: How do solids grow past the drift and fragmentation barriers?
Plain: Pebbles and boulders feel a headwind from the gas disk, so they lose orbital speed and spiral into the star in a few hundred orbits, and they shatter when they collide. Some process must form kilometer bodies faster than this.
Precise: Particles with Stokes number $\mathrm{St} = \Omega t_{\mathrm{stop}} \sim 0.1-1$ drift inward at $v_{r} \sim 2\,\mathrm{St}\,\eta\,v_{K}/(1+\mathrm{St}^{2})$ with $\eta v_{K} \sim 3-5x10^{3}\,\mathrm{cm/s}$ and collide at speeds above fragmentation thresholds of $\sim 10^{2}-10^{3}\,\mathrm{cm/s}$. Identify which pathway forms bound planetesimals of $1-100\,\mathrm{km}$ fast enough: streaming instability, trapping in pressure bumps or vortices, gravitational instability of the dust layer, or coagulation of porous ice aggregates.
Settled by: Disk models that form planetesimals in measured disk conditions and reproduce the size distribution and binarity of primordial Kuiper-belt objects.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: 1977