Properties of planetesimals from streaming instability and alternative channels

Characterize the properties of planetesimals formed via the streaming instability and alternative formation mechanisms—including the initial mass function and binary inclination distribution—and assess the robustness and discriminative power of current predictions given limitations in collisional physics and resolution.

Background

Simulations predict a top-heavy mass function and specific binary properties for streaming-instability planetesimals, with some consistency across Solar System tests (e.g., Kuiper Belt binaries). However, many simulations omit detailed collisional physics or cannot follow collapse to material density, and it remains unclear how robust these predictions are or how well they differentiate among formation pathways.

References

Open questions and problems that I would like to see resolved include, in no particular order: What are the properties of planetesimals that form from the streaming instability, and from alternative planetesimal formation channels? Important quantities such as the initial mass function, and the distribution of binary inclinations, have already been derived from simulations of the streaming instability. How robust these predictions are, and to what extent they discriminate between models, is not completely clear: few simulations include collisional physics or have the resolution to follow collapse to anything close to material density.

— Planet formation theory: an overview  (2412.11064 - Armitage, 2024) in Section “Some open questions”, Item 9

However, the applicability of standard SI thresholds to our models is uncertain because the dust is confined within a pressure bump, where the pressure gradient and radial drift velocity vary spatially. Dedicated pressure-bump simulations have shown that conventional local SI criteria are not sufficient to determine whether strong clumping occurs, particularly for small grains.

— Multi-wavelength Constraints on Dust Dynamics and Size Evolution in Protoplanetary Disk Rings. II. Observational Implications  (2609.16556 - Yang et al., 15 Sep 2026) in Section 4.3, “Gas-Dependent Constraints on α and v_frag”