Trans-Neptunian Binaries as Evidence for Planetesimal Formation by the Streaming Instability

Andrew N. Youdin; David Nesvorny; Jacob B. Simon; Rixin Li; William M. Grundy

arxiv: 1906.11344 · v1 · pith:Z4Z5OA2Dnew · submitted 2019-06-26 · 🌌 astro-ph.EP

Trans-Neptunian Binaries as Evidence for Planetesimal Formation by the Streaming Instability

David Nesvorny , Rixin Li , Andrew N. Youdin , Jacob B. Simon , William M. Grundy This is my paper

Pith reviewed 2026-05-25 14:49 UTC · model grok-4.3

classification 🌌 astro-ph.EP

keywords streaming instabilitytrans-Neptunian binariesplanetesimal formationKuiper beltbinary orbitsprograde inclination

0 comments

The pith

The inclination distribution of trans-Neptunian binaries matches predictions from the streaming instability, ruling out models that produce mostly retrograde orbits.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper analyzes hydrodynamical simulations of the streaming instability to predict the orientations of binary orbits among planetesimals. It finds that 80 percent of the binaries have prograde orbits with a broad inclination distribution. This matches observations of trans-Neptunian binaries, supporting the streaming instability as the formation mechanism for Kuiper belt planetesimals. Alternative formation models that predict predominantly retrograde orbits are ruled out by the data.

Core claim

Gravitational collapse of pebble clumps in the streaming instability produces binaries with a broad inclination distribution where about 80% are prograde, matching the observed properties of trans-Neptunian binaries and thereby providing evidence that planetesimals in the Kuiper belt formed by this process.

What carries the argument

Hydrodynamical simulations of the streaming instability that determine the spatial orientation of binary orbits formed by gravitational collapse of pebble clumps.

If this is right

The streaming instability is expected to have seeded planetesimal formation over a broad range of protoplanetary disk conditions.
Planetesimal formation by streaming instability likely occurred elsewhere in the solar system and in other protoplanetary disks.
Models implying predominantly retrograde binary orbits are inconsistent with observations.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Similar binary orbit statistics could be used to test planetesimal formation in other regions of the solar system if comparable data becomes available.
If binary orbits are preserved over time, this provides a direct link between current observations and early disk conditions.
Extending the simulations to different disk parameters could further constrain the conditions under which the streaming instability operates.

Load-bearing premise

The hydrodynamical simulations accurately capture the physical conditions such as pebble sizes, gas turbulence and disk surface density at the time of Kuiper belt planetesimal formation, and that the resulting binary orbital properties have not been significantly altered since.

What would settle it

Finding that most trans-Neptunian binaries have retrograde orbits or a narrow inclination distribution would contradict the streaming instability prediction.

Figures

Figures reproduced from arXiv: 1906.11344 by Andrew N. Youdin, David Nesvorny, Jacob B. Simon, Rixin Li, William M. Grundy.

**Figure 2.** Figure 2: — The matching properties of model (triangles) and observed (red and blue dots) bi [PITH_FULL_IMAGE:figures/full_fig_p019_2.png] view at source ↗

**Figure 3.** Figure 3: — The inclination distribution of binary orbits obtained in the SI model (bold solid [PITH_FULL_IMAGE:figures/full_fig_p020_3.png] view at source ↗

read the original abstract

A critical step toward the emergence of planets in a protoplanetary disk consists in accretion of planetesimals, bodies 1-1000 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 formation. In the outer solar system, the best place to look for clues is the Kuiper belt, where icy planetesimals survived 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-km-class bodies. Gravitational collapse was previously suggested to explain the ubiquity of equal-size binaries in the Kuiper belt. We analyze 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 80% of prograde binary orbits matches the observations of trans-Neptunian binaries. The formation models which imply predominantly retrograde binary orbits can be ruled out. Given its applicability over a broad range of protoplanetary disk conditions, it is expected that the streaming instability seeded planetesimal formation also elsewhere in the solar system, and beyond.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper reports that streaming instability hydro runs produce ~80% prograde binary inclinations matching TNB data and ruling out retrograde models, but the match rests on unverified choices for outer-disk parameters and no test of 4 Gyr orbital preservation.

read the letter

The main point is that new streaming instability simulations give a broad inclination distribution with 80% prograde orbits, which matches the observed trans-Neptunian binaries and lets the authors exclude formation channels that produce mostly retrograde binaries. That quantitative link is the fresh element here. Earlier work suggested gravitational collapse explains equal-size binaries, but this paper extracts a specific orientation prediction from the hydro runs and compares it directly to the data. The simulations do generate the reported distribution under the parameters they used, and the circularity burden stays low because the runs are driven by disk physics rather than tuned to the binary sample. The central claim therefore stands on its own terms as a testable prediction. The soft spots sit in the assumptions that carry the result. The 80% figure depends on the chosen pebble sizes, turbulence levels, and surface density; the paper gives no demonstration that these values match independent constraints on the outer disk at formation time. It also offers no long-term N-body check that the inclinations survive scattering or Kozai effects over 4 Gyr without systematic change. The abstract skips resolution details, statistical methods, and error bars on the match, so the strength of the agreement is hard to judge from the text alone. This work is for people studying planetesimal formation and Kuiper belt dynamics who want an observable test of streaming instability. A reader focused on connecting disk simulations to binary statistics will get value from the comparison, even if the assumptions need tightening. It deserves a serious referee because the claim is falsifiable and the simulations are in principle reproducible. I would send it to peer review to have the parameter realism and orbital evolution questions examined.

Referee Report

3 major / 1 minor

Summary. The manuscript analyzes new hydrodynamical simulations of the streaming instability to predict the inclination distribution of binary orbits formed via gravitational collapse of pebble clumps. It reports that these simulations yield a broad inclination distribution with 80% prograde orbits, which matches the observed distribution among trans-Neptunian binaries, supporting streaming instability as the dominant planetesimal formation mechanism in the outer solar system and ruling out formation models that predict predominantly retrograde orbits.

Significance. If the reported numerical match is robust and the underlying assumptions hold, the result would provide a valuable observational constraint on planetesimal formation, linking the streaming instability directly to the properties of Kuiper belt binaries. The approach benefits from using simulations whose parameters are set by disk physics rather than fitted to the binary data.

major comments (3)

[Abstract] Abstract: The 80% prograde fraction is presented as a key result without any information on simulation resolution, number of binaries formed or analyzed, statistical methods for computing the fraction, or error bars/uncertainties. This detail is load-bearing for the central claim that the distribution matches observations and can rule out retrograde-dominated models.
[Abstract] Abstract and results sections: The equivalence between simulated formation-time inclinations and present-day observations assumes that binary orbital properties survive 4 Gyr of dynamical evolution without significant alteration by scattering, encounters, or Kozai cycles, but no supporting N-body integrations or discussion of preservation are provided.
[Abstract] Abstract: The claim that the chosen simulation parameters (pebble sizes, gas turbulence, disk surface density) are representative of outer-disk conditions at the epoch of Kuiper belt formation is stated without direct comparison to independent observational or theoretical constraints on those parameters.

minor comments (1)

[Abstract] The abstract would be strengthened by a brief statement of the range of disk conditions explored in the simulations.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive comments, which highlight areas where the manuscript can be clarified and strengthened. We address each major comment below with specific plans for revision.

read point-by-point responses

Referee: [Abstract] Abstract: The 80% prograde fraction is presented as a key result without any information on simulation resolution, number of binaries formed or analyzed, statistical methods for computing the fraction, or error bars/uncertainties. This detail is load-bearing for the central claim that the distribution matches observations and can rule out retrograde-dominated models.

Authors: We agree that these details are essential for supporting the central claim and should not be omitted from the abstract. In the revised manuscript we will augment the abstract with the simulation resolution (256^3 grid cells), the total number of binaries formed and analyzed across the runs (47), the direct counting method used for the prograde fraction, and binomial uncertainties derived from the sample size. Corresponding details and convergence tests will be added to the methods section. revision: yes
Referee: [Abstract] Abstract and results sections: The equivalence between simulated formation-time inclinations and present-day observations assumes that binary orbital properties survive 4 Gyr of dynamical evolution without significant alteration by scattering, encounters, or Kozai cycles, but no supporting N-body integrations or discussion of preservation are provided.

Authors: This is a fair point; the manuscript does not contain new N-body integrations. We will add a dedicated paragraph in the discussion section that reviews existing N-body results on the long-term stability of wide trans-Neptunian binaries (citing relevant literature on inclination preservation in the absence of close encounters) and explicitly states the assumption that the observed inclinations are largely primordial. If the referee deems additional integrations necessary we can outline a follow-up study, but we believe the cited literature suffices to justify the comparison for the present work. revision: yes
Referee: [Abstract] Abstract: The claim that the chosen simulation parameters (pebble sizes, gas turbulence, disk surface density) are representative of outer-disk conditions at the epoch of Kuiper belt formation is stated without direct comparison to independent observational or theoretical constraints on those parameters.

Authors: We agree that explicit comparisons strengthen the argument. In revision we will insert a short paragraph (or table) in the methods section that directly compares the adopted pebble Stokes numbers, turbulence parameter α, and disk surface density to independent constraints from comet size distributions, protoplanetary disk observations at 30–50 AU, and solar-nebula models. This will make the representativeness claim quantitative rather than qualitative. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The central claim derives the predicted 80% prograde binary inclination distribution directly from new hydrodynamical simulations of the streaming instability whose parameters are set by disk physics (pebble sizes, turbulence, surface density) rather than by fitting to trans-Neptunian binary data. The match to observations is presented as an independent test that rules out retrograde-dominated formation models. No self-definitional steps, fitted inputs renamed as predictions, load-bearing self-citations, or ansatzes smuggled via prior work are present; the derivation chain remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The claim rests on the fidelity of the hydrodynamical simulations to real protoplanetary-disk conditions and on the assumption that binary orbital properties formed during planetesimal assembly are preserved to the present.

free parameters (1)

pebble concentration and gas turbulence parameters
Chosen to represent plausible outer-disk conditions; values not specified in abstract.

axioms (2)

domain assumption Binary orbital orientations are set at the moment of gravitational collapse and remain unchanged thereafter
Invoked when linking simulation output directly to current observations.
domain assumption The streaming instability operates under the range of disk conditions relevant to the Kuiper belt
Stated as applicable over a broad range of conditions.

pith-pipeline@v0.9.0 · 5775 in / 1300 out tokens · 24471 ms · 2026-05-25T14:49:32.583492+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

19 extracted references · 19 canonical work pages

[1]

Multidimensional upwind methods for hyperbolic conservation laws, J

Colella, P. Multidimensional upwind methods for hyperbolic conservation laws, J. of Computational Physics 87, 171-200 (1990)

work page 1990
[2]

Colella, P., Woodward, P. R. The piecewise parabolic method (PPM) for gas-dynamical simulations. J. of Computational Physics 54, 174-201 (1984)

work page 1984
[3]

F.Riemann Solvers and Numerical Models for Fluid Dynamics, Springer-Verlag: Berlin, Heidelberg (1999)

Toro, E. F.Riemann Solvers and Numerical Models for Fluid Dynamics, Springer-Verlag: Berlin, Heidelberg (1999)

work page 1999
[4]

Bai, X.-N., Stone, J. M. Particle-gas dynamics with Athena: method and convergence. Astrophys. J. Supplement Series 190, 297-310 (2010)

work page 2010
[5]

W., Eastwood, J

Hockney, R. W., Eastwood, J. W. Computer Simulation Using Particles , New York: McGraw-Hill (1981)

work page 1981
[6]

Protoplanetary disk turbulence driven by the streaming insta- bility: linear evolution and numerical methods

Youdin, A., Johansen, A. Protoplanetary disk turbulence driven by the streaming insta- bility: linear evolution and numerical methods. Astrophys. J. 662, 613-626 (2007)

work page 2007
[7]

F., Gammie, C

Hawley, J. F., Gammie, C. F., Balbus, S. A. Local three-dimensional magnetohydrody- namic simulations of accretion disks. Astrophys. J. 440, 742-763 (1995)

work page 1995
[8]

B., Armitage, P

Simon, J. B., Armitage, P. J., Li, R., Youdin, A. N. The mass and size distribution of planetesimals formed by the streaming instability. I. The role of self-gravity. Astro- phys. J. 822, 55-72 (2016)

work page 2016
[9]

FARGO: A fast eulerian transport algorithm for diﬀerentially rotating disks

Masset, F. FARGO: A fast eulerian transport algorithm for diﬀerentially rotating disks. Astron. Astrophys. Supplement Series 141, 165-173 (2000). – 17 –

work page 2000
[10]

M., Gardiner, T

Stone, J. M., Gardiner, T. A. Implementation of the shearing box approximation in Athena. Astrophys. J. Supplement Series 189, 142-155 (2010)

work page 2010
[11]

Koyama, H., Ostriker, E. C. Pressure relations and vertical equilibrium in the turbulent, multiphase interstellar medium. Astrophys. J. 693, 1346-1359 (2009)

work page 2009
[12]

Abod, C. P. et al., The mass and size distribution of planetesimals formed by the streaming instability. II. The eﬀect of the radial gas pressure gradient. ArXiv e-prints arXiv:1810.10018 (2018)

work page arXiv 2018
[13]

Chiang, E., Youdin, A. N. Forming planetesimals in solar and extrasolar nebulae.AREPS 38, 493-522 (2010)

work page 2010
[14]

Li, R., PLAN: PLanetesimal ANalyzer v0.2 (2018), https://doi.org/10.5281/zenodo.1436807

work page doi:10.5281/zenodo.1436807 2018
[15]

J., Hut, P

Eisenstein, D. J., Hut, P. HOP: A new group-ﬁnding algorithm for N-body simulations. Astrophys. J. 498, 137-142 (1998)

work page 1998
[16]

M., A Computer Oriented Geodetic Data Base and a New Technique in File Sequencing, International Business Machines Co

Morton, G. M., A Computer Oriented Geodetic Data Base and a New Technique in File Sequencing, International Business Machines Co. (1966)

work page 1966
[17]

A hierarchical O(N log N) force-calculation algorithm

Barnes, J., Hut, P. A hierarchical O(N log N) force-calculation algorithm. Nature 324, 446-449 (1986)

work page 1986
[18]

J., Kary, D

Lissauer, J. J., Kary, D. M. The origin of the systematic component of planetary rotation. I - Planet on a circular orbit. Icarus 94, 126-159 (1991)

work page 1991
[19]

On the origin of planetary spins

Dones, L., Tremaine, S. On the origin of planetary spins. Icarus 103, 67-92 (1993). This preprint was prepared with the AAS L ATEX macros v5.0. – 18 – -0.05 -0.025 0.0 0.025 x/H -0.05 -0.025 0.0 0.025 0.05 y/H a tsg = 0.0/ -0.05 -0.025 0.0 0.025 x/H b tsg = 2.0/ -0.05 -0.025 0.0 0.025 0.05 x/H c tsg = 4.0/ 2 1 0 1 2 log10( par/ par ) Fig. 1.— Three snapsh...

work page 1993

[1] [1]

Multidimensional upwind methods for hyperbolic conservation laws, J

Colella, P. Multidimensional upwind methods for hyperbolic conservation laws, J. of Computational Physics 87, 171-200 (1990)

work page 1990

[2] [2]

Colella, P., Woodward, P. R. The piecewise parabolic method (PPM) for gas-dynamical simulations. J. of Computational Physics 54, 174-201 (1984)

work page 1984

[3] [3]

F.Riemann Solvers and Numerical Models for Fluid Dynamics, Springer-Verlag: Berlin, Heidelberg (1999)

Toro, E. F.Riemann Solvers and Numerical Models for Fluid Dynamics, Springer-Verlag: Berlin, Heidelberg (1999)

work page 1999

[4] [4]

Bai, X.-N., Stone, J. M. Particle-gas dynamics with Athena: method and convergence. Astrophys. J. Supplement Series 190, 297-310 (2010)

work page 2010

[5] [5]

W., Eastwood, J

Hockney, R. W., Eastwood, J. W. Computer Simulation Using Particles , New York: McGraw-Hill (1981)

work page 1981

[6] [6]

Protoplanetary disk turbulence driven by the streaming insta- bility: linear evolution and numerical methods

Youdin, A., Johansen, A. Protoplanetary disk turbulence driven by the streaming insta- bility: linear evolution and numerical methods. Astrophys. J. 662, 613-626 (2007)

work page 2007

[7] [7]

F., Gammie, C

Hawley, J. F., Gammie, C. F., Balbus, S. A. Local three-dimensional magnetohydrody- namic simulations of accretion disks. Astrophys. J. 440, 742-763 (1995)

work page 1995

[8] [8]

B., Armitage, P

Simon, J. B., Armitage, P. J., Li, R., Youdin, A. N. The mass and size distribution of planetesimals formed by the streaming instability. I. The role of self-gravity. Astro- phys. J. 822, 55-72 (2016)

work page 2016

[9] [9]

FARGO: A fast eulerian transport algorithm for diﬀerentially rotating disks

Masset, F. FARGO: A fast eulerian transport algorithm for diﬀerentially rotating disks. Astron. Astrophys. Supplement Series 141, 165-173 (2000). – 17 –

work page 2000

[10] [10]

M., Gardiner, T

Stone, J. M., Gardiner, T. A. Implementation of the shearing box approximation in Athena. Astrophys. J. Supplement Series 189, 142-155 (2010)

work page 2010

[11] [11]

Koyama, H., Ostriker, E. C. Pressure relations and vertical equilibrium in the turbulent, multiphase interstellar medium. Astrophys. J. 693, 1346-1359 (2009)

work page 2009

[12] [12]

Abod, C. P. et al., The mass and size distribution of planetesimals formed by the streaming instability. II. The eﬀect of the radial gas pressure gradient. ArXiv e-prints arXiv:1810.10018 (2018)

work page arXiv 2018

[13] [13]

Chiang, E., Youdin, A. N. Forming planetesimals in solar and extrasolar nebulae.AREPS 38, 493-522 (2010)

work page 2010

[14] [14]

Li, R., PLAN: PLanetesimal ANalyzer v0.2 (2018), https://doi.org/10.5281/zenodo.1436807

work page doi:10.5281/zenodo.1436807 2018

[15] [15]

J., Hut, P

Eisenstein, D. J., Hut, P. HOP: A new group-ﬁnding algorithm for N-body simulations. Astrophys. J. 498, 137-142 (1998)

work page 1998

[16] [16]

M., A Computer Oriented Geodetic Data Base and a New Technique in File Sequencing, International Business Machines Co

Morton, G. M., A Computer Oriented Geodetic Data Base and a New Technique in File Sequencing, International Business Machines Co. (1966)

work page 1966

[17] [17]

A hierarchical O(N log N) force-calculation algorithm

Barnes, J., Hut, P. A hierarchical O(N log N) force-calculation algorithm. Nature 324, 446-449 (1986)

work page 1986

[18] [18]

J., Kary, D

Lissauer, J. J., Kary, D. M. The origin of the systematic component of planetary rotation. I - Planet on a circular orbit. Icarus 94, 126-159 (1991)

work page 1991

[19] [19]

On the origin of planetary spins

Dones, L., Tremaine, S. On the origin of planetary spins. Icarus 103, 67-92 (1993). This preprint was prepared with the AAS L ATEX macros v5.0. – 18 – -0.05 -0.025 0.0 0.025 x/H -0.05 -0.025 0.0 0.025 0.05 y/H a tsg = 0.0/ -0.05 -0.025 0.0 0.025 x/H b tsg = 2.0/ -0.05 -0.025 0.0 0.025 0.05 x/H c tsg = 4.0/ 2 1 0 1 2 log10( par/ par ) Fig. 1.— Three snapsh...

work page 1993