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Trans-Neptunian binaries as evidence for planetesimal formation by the streaming instability


A critical step toward the emergence of planets in a protoplanetary disk is the accretion of planetesimals, bodies 1–1,000 km in size, from smaller disk constituents. This process is poorly understood, partly because we lack good observational constraints on the complex physical processes that contribute to planetesimal formation1. In the outer Solar System, the best place to look for clues is the Kuiper belt, where icy planetesimals survive to this day. Here we report evidence that Kuiper belt planetesimals formed by the streaming instability, a process in which aerodynamically concentrated clumps of pebbles gravitationally collapse into 100-kilometre-class bodies2. Gravitational collapse has previously been suggested to explain the ubiquity of equal-sized binaries in the Kuiper belt3,4,5. We analyse new hydrodynamical simulations of the streaming instability to determine the model expectations for the spatial orientation of binary orbits. The predicted broad inclination distribution with approximately 80% of prograde binary orbits matches the observations of trans-Neptunian binaries6. The formation models that imply predominantly retrograde binary orbits (for example, ref. 7) can be ruled out. Given its applicability over a wide range of protoplanetary disk conditions8, it is expected that the streaming instability also seeded planetesimal formation elsewhere in the Solar System, and beyond.

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Fig. 1: Three snapshots from our 3D simulation of the SI where the nonlinear particle clumping triggers gravitational collapse into planetesimals.
Fig. 2: The matching properties of model (triangles) and observed (red and blue dots) binary planetesimals.
Fig. 3: The inclination distribution of binary orbits obtained in the SI model (bold solid line) matches observations of trans-Neptunian binaries (bold dashed line).

Data availability

The data that support the plots within this Letter and other findings of this study are available from the corresponding author on reasonable request.

Code availability

The Athena code is available on GitHub ( The PLAN code is available on Zenodo44.


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The work of D.N. was funded by the NASA Emerging Worlds programme. R.L. acknowledges support from NASA grant NNX16AP53H. A.N.Y. acknowledges support from NASA through grant NNX17AK59G and the NSF through grant 1616929. The funding sources of J.B.S. are NASA grants NNX13AI58G, NNX16AB42G, 80NSSC18K0640 and 80NSSC18K0597. W.M.G.’s contribution was supported in part by NASA Keck PI Data Awards, administered by the NASA Exoplanet Science Institute, and in part by data analysis grants from the Space Telescope Science Institute (STScI), operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under NASA contract NAS 5-26555.

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Authors and Affiliations



D.N. suggested a comparison of clump obliquities with binary orbit inclinations and prepared the manuscript for publication. R.L. ran one of the Athena simulations and performed data analyses with PLAN. A.N.Y. developed scaling relations for planetesimal mass estimates. J.B.S. ran two of the Athena simulations. W.M.G. provided the data on trans-Neptunian binaries. All authors contributed to the interpretation of the results and writing of this letter.

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Correspondence to David Nesvorný.

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Peer review information: Nature Astronomy thanks J. J. Kavelaars and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary text, Supplementary Video caption, Supplementary references, Supplementary Table 1 and Supplementary Figs. 1–5.

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Nesvorný, D., Li, R., Youdin, A.N. et al. Trans-Neptunian binaries as evidence for planetesimal formation by the streaming instability. Nat Astron 3, 808–812 (2019).

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