During the initial stages of planet formation in circumstellar gas disks, dust grains collide and build up larger and larger bodies1. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem2: boulders are expected to stick together poorly3, and to spiral into the protostar in a few hundred orbits owing to a ‘headwind’ from the slower rotating gas4. Gravitational collapse of the solid component has been suggested to overcome this barrier1,5,6. But even low levels of turbulence will inhibit sedimentation of solids to a sufficiently dense midplane layer2,7, and turbulence must be present to explain observed gas accretion in protostellar disks8. Here we report that boulders can undergo efficient gravitational collapse in locally overdense regions in the midplane of the disk. The boulders concentrate initially in transient high pressure regions in the turbulent gas9, and these concentrations are augmented a further order of magnitude by a streaming instability10,11,12 driven by the relative flow of gas and solids. We find that gravitationally bound clusters form with masses comparable to dwarf planets and containing a distribution of boulder sizes. Gravitational collapse happens much faster than radial drift, offering a possible path to planetesimal formation in accreting circumstellar disks.
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This collaboration was made possible through the support of the Annette Kade Graduate Student Fellowship Program at the American Museum of Natural History. J.S.O. was supported by the US National Science Foundation, as was M.-M.M.L. in part. We thank J. Cuzzi for discussion about the role of cooling in the gravitational collapse.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
This file contains Supplementary Discussion; Supplementary Notes with Supplementary Figures 1-27 and additional references. (PDF 1763 kb)
This file contains Supplementary Video 1. The video follows the column density of dust particles from the onset of self-gravity (at t=0) to where gravitationally bound clumps have formed. The inset shows an enlargement around the densest point in the simulation. Clear accretion features are visible as the clump attracts solid material from the surroundings. (MOV 9878 kb)
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Johansen, A., Oishi, J., Low, MM. et al. Rapid planetesimal formation in turbulent circumstellar disks. Nature 448, 1022–1025 (2007). https://doi.org/10.1038/nature06086
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