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Balance between pebbles and planetesimals

Determine the physical processes and disk conditions that set the partition of aerodynamically coupled solids (“pebbles,” typically with Stokes number around 0.1) between planetesimal formation via the streaming instability and pebble accretion onto growing protoplanets, recognizing that the available pebble reservoir can only be consumed once.

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Background

Earlier sections outline two key growth channels: planetesimal formation initiated by the streaming instability and core growth via pebble accretion. Both depend critically on the availability and properties of aerodynamically coupled solids. The author emphasizes that pebbles are a finite resource and that how they are apportioned between these pathways is a central unresolved issue that impacts subsequent planetary growth rates and architectures.

References

Open questions and problems that I would like to see resolved include, in no particular order: What sets the balance between pebbles and planetesimals? Aerodynamically coupled particles, especially those with τS ∼ 0.1, are valuable currency in contemporary theory, both for planetesimal formation via the streaming instability and for pebble accretion. Pebbles can only be spent once, however, and it is not clear how the balance is determined.

— Planet formation theory: an overview (2412.11064 - Armitage, 15 Dec 2024) in Section “Some open questions”, Item 1