Dust dynamics in disk dust traps and late planetesimal formation

¹ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
² Collège de France, 11 Pl. Berthelot, 75005 Paris, France

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 29 January 2026
Accepted: 13 May 2026

Abstract

Context. Streaming instability (SI) is currently the leading model for planetesimal formation in protoplanetary disks, but it typically operates on the radial drift timescale of solids toward the star, that is, within approximately the first million years. In the Solar System, however, some planetesimals (i.e., the parent bodies of chondritic meteorites) formed 2–4 Myr after disk formation, implying that dust must have been retained in the disk for extended periods. Pressure bumps provide an efficient mechanism for trapping dust. However, dust trapping alone does not guarantee planetesimal formation: even modest levels of gas turbulence can inhibit strong vertical settling and radial concentration, preventing the dust density from reaching the threshold required for gravitational collapse. This motivates the exploration of alternative dust-gas instabilities, such as the dusty Rossby wave instability (DRWI), which was first studied in 2D shearing-box simulations.

Aims. We aim to investigate the viability of such alternative instabilities in global disk simulations under realistic physical conditions.

Methods. We used the numerical code fargOCA, where the treatment of dust as a pressureless fluid was recently implemented. We first recovered the results of previous 2D shearing-box simulations using global 2D disk simulations and extended the analysis to fully 3D disks in both viscous and inviscid regimes.

Results. We reproduced prior 2D results and extended them by characterizing the dust clumping produced by the DRWI in a viscous disk (α = 10⁻⁴). We find that this instability does not develop in fully 3D viscous disks, quenched by high-z gas layers that remain unperturbed due to the settling of dust near the midplane. Motivated by this suppression, we explored the inviscid limit and found that multiple dust subrings form, concentrating solids into several thin ring-like structures. These structures would remain unresolved in observations and would therefore appear as a single radially broad and vertically thin ring. This explains the geometry of the rings observed in protoplanetary disks without any need to invoke anisotropic turbulence. With regard to planetesimal formation, dust concentrations in the subrings might remain smaller than the threshold for gravitational collapse. However, gas photoevaporation enhances dust settling and (partially) radial concentration, eventually triggering the formation of dust clumps of increasingly large density, in both the viscous and inviscid cases.

Conclusions. We conclude that planetesimal formation within dust-trapping pressure bumps is favored in very low-viscosity disks at late evolutionary stages, when sufficient gas has been removed by photoevaporation. This result is consistent with the inferred late formation of the parent bodies of chondritic meteorites in the Solar System.