The formation of Kuiper-belt binaries through exchange reactions


Recent observations1,2,3,4,5,6,7,8 have revealed that an unexpectedly high fraction—a few per cent—of the trans-Neptunian objects (TNOs) that inhabit the Kuiper belt are binaries. The components have roughly equal masses, with very eccentric orbits that are wider than a hundred times the radius of the primary. Standard theories of binary asteroid formation tend to produce close binaries with circular orbits, so two models have been proposed9,10 to explain the unique characteristics of the TNOs. Both models, however, require extreme assumptions regarding the size distribution of the TNOs. Here we report a mechanism that is capable of producing binary TNOs with the observed properties during the early stages of their formation and growth. The only required assumption is that the TNOs were initially formed through gravitational instabilities11 in the protoplanetary dust disk. The basis of the mechanism is an exchange reaction in which a binary whose primary component is much more massive than the secondary interacts with a third body, whose mass is comparable to that of the primary. The low-mass secondary component is ejected and replaced by the third body in a wide but eccentric orbit.

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Figure 1: Paths of formation of binaries.
Figure 2: An example of a binary–single-body exchange interaction.
Figure 3: Normalized differential cross-sections for the formation of a ‘massive–massive’ binary.
Figure 4: Distribution of orbital properties of ‘massive–massive’ binaries formed in our scattering experiments.


  1. 1

    Burns, J. A. Two Bodies Are Better Than One. Science 297, 942–943 (2002)

    ADS  CAS  Article  Google Scholar 

  2. 2

    Veillet, C. et al. The binary Kuiper-belt object 1998 WW 31. Nature 416, 711–713 (2002)

    ADS  CAS  Article  Google Scholar 

  3. 3

    Margot, J.-L. Worlds of mutual motion. Nature 416, 694–695 (2002)

    ADS  CAS  Article  Google Scholar 

  4. 4

    Noll, K. S. et al. Detection of two binary trans-Neptunian objects, 1997 CQ29 and 2000 CF105, with the Hubble Space Telescope. Astron. J. 124, 3424–3429 (2002)

    ADS  Article  Google Scholar 

  5. 5

    Schaller, E. L. & Brown, M. E. A deep Keck search for binary Kuiper Belt objects. DPS 35th Meeting Abstr. no. 39.20 (American Astronomical Society, Washington DC, 2003)

  6. 6

    Noll, K. S. et al. Discovery of transneptunian binaries with HST. DPS 35th Meeting Abstr. no. 49.70 (American Astronomical Society, Washington DC, 2003)

  7. 7

    Noll, K. S. et al. 2001 QC_298. IAU Circ. No. 8034 (2002)

  8. 8

    Noll, K. S. & Stephens, D. C. 1999 RZ_253. IAU Circ. No. 8143 (2003)

  9. 9

    Weidenschilling, S. J. On the origin of binary transneptunian objects. Icarus 160, 212–215 (2002)

    ADS  Article  Google Scholar 

  10. 10

    Goldreich, P., Lithwick, Y. & Sari, R. Formation of Kuiper-belt binaries by dynamical friction and three-body encounters. Nature 420, 643–646 (2002)

    ADS  CAS  Article  Google Scholar 

  11. 11

    Goldreich, P. & Ward, W. R. The formation of planetesimals. Astrophys. J. 183, 1051–1062 (1973)

    ADS  Article  Google Scholar 

  12. 12

    Merline, W. J. et al. in Asteroids III (eds Bottke, W. F., Cellino, A., Paolicchi, P. & Binzel, R. P.) 273–287, (Lunar and Planetary Institute, Houston, 2003)

    Google Scholar 

  13. 13

    Hartmann, W. K. & Davis, D. R. Satellite-sized planetesimals and lunar origin. Icarus 24, 504–515 (1975)

    ADS  Article  Google Scholar 

  14. 14

    Kokubo, E., Ida, S. & Makino, J. Evolution of a circumterrestrial disk and formation of a single moon. Icarus 148, 419–436 (2000)

    ADS  Article  Google Scholar 

  15. 15

    Durda, D. D., Bottke, W. F., Asphaug, E. & Richardson, D. C. The formation of asteroid satellites: Numerical simulations using SPH and N-body models. Bull. Am. Astron. Soc. 33, 1134 (2001)

    ADS  Google Scholar 

  16. 16

    Durda, D. D. et al. The formation of asteroid satellites in catastrophic impacts: Results from numerical simulations. Lunar Planet Sci. XXXIV Abstr. no. 1943 (Lunar and Planetary Institute, Houston, 2003)

  17. 17

    Fabian, A. C., Pringle, J. E. & Rees, M. J. Tidal capture formation of binary systems and X-ray sources in globular clusters. Mon. Not. R. Astron. Soc. 172, 15–18 (1975)

    ADS  Article  Google Scholar 

  18. 18

    Heggie, D. C. Binary evolution in stellar dynamics. Mon. Not. R. Astron. Soc. 173, 729–787 (1975)

    ADS  Article  Google Scholar 

  19. 19

    Kokubo, E. & Ida, S. Oligarchic growth of protoplanets. Icarus 131, 171–178 (1998)

    ADS  Article  Google Scholar 

  20. 20

    Makino, J., Fukushige, T., Funato, Y. & Kokubo, E. On the mass distribution of planetesimals in the early runaway stage. New Astron. 3, 411–416 (1998)

    ADS  Article  Google Scholar 

  21. 21

    Wetherill, G. W. Formation of the earth. Annu. Rev. Earth Planet. Sci. 18, 205–256 (1990)

    ADS  Article  Google Scholar 

  22. 22

    Kenyon, S. J. & Luu, J. X. Accretion in the Early Kuiper Belt. I. Coagulation and velocity evolution. Astron. J. 115, 2125–2160 (1998)

    ADS  Article  Google Scholar 

  23. 23

    Spitzer, L. Dynamical evolution of globular clusters. (Princeton Univ. Press, Princeton, NJ, 1987)

  24. 24

    Hut, P. Hard binary-single star scattering cross sections for equal masses. Astrophys. J. Suppl. 55, 301–317 (1984)

    ADS  Article  Google Scholar 

  25. 25

    Hut, P. & Inagaki, S. Globular cluster evolution with finite-size stars—Cross sections and reaction rates. Astrophys. J. 298, 502–520 (1985)

    ADS  Article  Google Scholar 

  26. 26

    Heggie, D. C. & Hut, P. The Gravitational Million-Body Problem Ch. 23 (Cambridge Univ. Press, Cambridge, 2003)

    Google Scholar 

  27. 27

    McMillan, S. & Hut, P. Binary-single-star scattering. VI. Automatic determination of interaction cross sections. Astrophys. J. 467, 348–358 (1996)

    ADS  Article  Google Scholar 

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We acknowledge comments on our manuscript by P. Goldreich, R. Rafikov, R. Sari, K. S. Noll and D. Durda.

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Correspondence to Yoko Funato.

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Funato, Y., Makino, J., Hut, P. et al. The formation of Kuiper-belt binaries through exchange reactions. Nature 427, 518–520 (2004).

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