How do solids grow past the drift and fragmentation barriers?
In plain words
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 statement
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.
What would settle it
Disk models that form planetesimals in measured disk conditions and reproduce the size distribution and binarity of primordial Kuiper-belt objects.
Related problems
- More general than What is the birth size distribution of planetesimals?
- More general than When does the streaming instability collapse pebbles into planetesimals?