Turbulence sets the length scale for planetesimal formation: Local 2D simulations of streaming instability and planetesimal formation

Abstract

The trans-Neptunian object 2014 MU69, named Arrokoth, is the most recent evidence that planetesimals did not form by successive collisions of smaller objects, but by the direct gravitational collapse of a pebble cloud. But what process sets the physical scales on which this collapse may occur? Star formation has the Jeans mass, that is when gravity is stronger than thermal pressure, helping us to understand the mass of our sun. But what controls mass and size in the case of planetesimal formation? Both asteroids and Kuiper belt objects show a kink in their size distribution at 100 km. Here we derive a gravitational collapse criterion for a pebble cloud to fragment to planetesimals, showing that a critical mass is needed for the clump to overcome turbulent diffusion. We successfully tested the validity of this criterion in direct numerical simulations of planetesimal formation triggered by the streaming instability. Our result can, therefore, explain the sizes for planetesimals found forming in streaming instability simulations in the literature, while not addressing the detailed size distribution. We find that the observed characteristic diameters of $\sim$ 100 km correspond to the critical mass of a pebble cloud set by the strength of turbulent diffusion stemming from streaming instability for a wide region of a solar nebula model from 2 - 60 au, with a tendency to allow for smaller objects at distances beyond and at late times, when the nebula gas gets depleted.

30 pages, 16 figures, 3 movies, ApJ in press

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Cited by in corpus (16)

• Solution to the debris disc mass problem: planetesimals are born small?
• How dust fragmentation may be beneficial to planetary growth by pebble accretion
• Testing the Jeans, Toomre and Bonnor-Ebert concepts for planetesimal formation: 3D streaming instability simulations of diffusion regulated formation of planetesimals
• Growing and Trapping Pebbles with Fragile Collisions of Particles in Protoplanetary Disks
• A re-assessment of the Kuiper belt size distribution for sub-kilometer objects, revealing collisional equilibrium at small sizes
• A $75\%$ Occurrence Rate of Debris Discs around F stars in the $\beta$ Pic Moving Group
• Polydisperse Streaming Instability III. Dust evolution encourages fast instability
• Constraining planetesimal stirring: how sharp are debris disc edges?
• How planets grow by pebble accretion IV: Envelope opacity trends from sedimenting dust and pebbles
• A Smoking Gun for Planetesimal Formation: Charge Driven Growth into a New Size Range
• Constraints on planetesimal accretion inferred from particle-size distribution in CO chondrites
• The formation of wide exoKuiper belts from migrating dust traps
• Possible Ribose Synthesis in Carbonaceous Planetesimals
• Particle Dynamics in 3D Self-gravitating Disks II: Strong Gas Accretion and Thin Dust Disks
• Vertical settling of pebbles in turbulent circumbinary discs and the in situ formation of circumbinary planets
• The radial structure of planetary bodies formed by the streaming instability