ASTRO
In the literature: open
What is the birth size distribution of planetesimals?
In plain words
If planetesimals form by sudden gravitational collapse, they should be born big, around 100 kilometers. The size mix at birth determines how fast planets can grow afterwards.
Precise statement
Predict and measure $\mathrm{d}N/\mathrm{d}D$ of planetesimals at formation. Streaming-instability collapse simulations give a top-heavy distribution peaking near $D \sim 100\,\mathrm{km}$; test against the size distribution and binary fraction of cold classical Kuiper-belt objects and against asteroid-belt constraints. The answer is $\mathrm{d}N/\mathrm{d}D$ at birth with its dependence on disk conditions.
What would settle it
Kuiper-belt surveys complete to $D \sim 1-10\,\mathrm{km}$ plus collapse simulations that resolve fragmentation into binaries and smaller bodies.
Related problems
- Special case of How do solids grow past the drift and fragmentation barriers?