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Title: What is the birth size distribution of planetesimals?
Plain: 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: 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.
Settled by: Kuiper-belt surveys complete to $D \sim 1-10\,\mathrm{km}$ plus collapse simulations that resolve fragmentation into binaries and smaller bodies.
Kind: well-posed
Answer type: value
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)