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The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration


The ‘dynamically cold Kuiper belt’ consists of objects on low-inclination orbits between 40 and 50 au from the Sun. It currently contains material totalling less than a tenth the mass of the Earth1,2, which is surprisingly low because, according to accretion models3,4, the objects would not have grown to their present size unless the cold Kuiper belt originally contained tens of Earth masses of solids. Although several mechanisms have been proposed to produce the observed mass depletion, they all have significant limitations5. Here we show that the objects currently observed in the dynamically cold Kuiper belt were most probably formed within 35 au and were subsequently pushed outward by Neptune's 1:2 mean motion resonance during its final phase of migration. Combining our mechanism with previous work6,7, we conclude that the entire Kuiper belt formed closer to the Sun and was transported outward during the final stages of planet formation.

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Figure 1: The semi-major axis versus eccentricity distribution for various ‘cold’ populations.
Figure 2: The evolution of eccentricity of particles in the 1:2 MMR in various physical situations.
Figure 3: The power spectrum of the periapse precession of Neptune's orbit that illustrates the origin of our new secular resonance.


  1. Trujillo, C. A., Jewitt, D. C. & Luu, J. X. Properties of the Trans-Neptunian belt: statistics from the Canada-France-Hawaii telescope survey. Astron. J. 122, 457–473 (2001)

    ADS  Article  Google Scholar 

  2. Gladman, B. et al. The structure of the Kuiper belt: Size distribution and radial extent. Astron. J. 122, 1051–1066 (2001)

    ADS  Article  Google Scholar 

  3. Stern, S. A. & Colwell, J. E. Accretion in the Edgeworth-Kuiper belt: forming 100–1000 km radius bodies at 30 au and beyond. Astron. J. 114, 841–849 (1997)

    ADS  Article  Google Scholar 

  4. Kenyon, S. J. & Luu, J. X. Accretion in the early Kuiper belt. I. Coagulation and velocity evolution. Astron. J. 115, 2136–2160 (1998)

    ADS  Article  Google Scholar 

  5. Morbidelli, A. & Brown, M. E. The Kuiper belt and the primordial evolution of the solar system. In Comets II (eds Festou, M., Keller, H. U. & Weaver, H.) (Univ. Arizona Press, Tucson, in the press)

  6. Gomes, R. S. The origin of the Kuiper belt high inclination population. Icarus 161, 404–418 (2003)

    ADS  Article  Google Scholar 

  7. Malhotra, R. The origin of Pluto's orbit: implications for the Solar System beyond Neptune. Astron. J. 110, 420–432 (1995)

    ADS  Article  Google Scholar 

  8. Jewitt, D. C. & Luu, J. X. Discovery of the candidate Kuiper belt object 1992 QB1. Nature 362, 730–732 (1993)

    ADS  Article  Google Scholar 

  9. Brown, M. The inclination distribution of the Kuiper belt. Astron. J. 121, 2804–2814 (2001)

    ADS  Article  Google Scholar 

  10. Levison, H. F. & Stern, S. A. On the size dependence of the inclination distribution of the main Kuiper belt. Astron. J. 121, 1730–1735 (2001)

    ADS  Article  Google Scholar 

  11. Trujillo, C. A. & Brown, M. E. A correlation between inclination and color in the classical Kuiper belt. Astrophys. J. 566, 125–128 (2002)

    ADS  Article  Google Scholar 

  12. Trujillo, C. A. & Brown, M. E. The radial distribution of the Kuiper belt. Astrophys. J. 554, 95–98 (2001)

    ADS  Article  Google Scholar 

  13. Allen, R. L., Bernstein, G. M. & Malhotra, R. Observational limits on a distant cold Kuiper belt. Astron. J. 124, 2949–2954 (2002)

    ADS  Article  Google Scholar 

  14. Ida, S., Larwood, J. & Burkert, A. Evidence for early stellar encounters in the orbital distribution of Edgeworth-Kuiper belt objects. Astrophys. J. 528, 351–356 (2000)

    ADS  Article  Google Scholar 

  15. Weidenschilling, S. Formation of planetesimals/cometesimals in the solar nebula. In Comets II (eds Festou, M., Keller, H. U. & Weaver, H.) (Univ. Arizona Press, Tucson, in the press)

  16. Hollenbach, D. & Adams, F. C. Dispersal of disks around young stars: constraints on Kuiper belt formation. In Debris Disks and the Formation of Planets (eds Caroff, L. & Backman, D.) (ASP, San Francisco, in the press)

  17. Stone, J. M., Gammie, C. F., Balbus, S. A. & Hawley, J. F. in Protostars and Planets IV (eds Mannings, V., Boss, A. P. & Russell, S. S.) 589–612 (Univ. Arizona Press, Tucson, 1998)

    Google Scholar 

  18. Fernández, J. A. & Ip, W. H. Orbital expansion and resonant trapping during the late accretion stages of the outer planets. Planet. Space Sci. 44, 431–439 (1996)

    ADS  Article  Google Scholar 

  19. Morbidelli, A. & Levison, H. F. Kuiper belt interlopers. Nature 422, 30–31 (2003)

    ADS  CAS  Article  Google Scholar 

  20. Duncan, M. & Levison, H. A scattered disk of icy objects and the origin of Jupiter-family comets. Science 276, 1670–1672 (1997)

    ADS  CAS  Article  Google Scholar 

  21. Luu, J. et al. A new dynamical class in the trans-Neptunian Solar System. Nature 387, 573 (1997)

    ADS  CAS  Article  Google Scholar 

  22. Gomes, R. S., Morbidelli, A. & Levison, H. F. Planetary migration in a planetesimal disk: why did Neptune stop at 30 AU? Icarus (submitted)

  23. Duncan, M. J., Levison, H. F. & Lee, M.-H. A multiple timestep symplectic algorithm for integrating close encounters. Astron. J. 116, 2067–2077 (1998)

    ADS  Article  Google Scholar 

  24. Wisdom, J. & Holman, M. Symplectic maps for the n-body problem. Astron. J. 102, 1528–1538 (1991)

    ADS  Article  Google Scholar 

  25. Hahn, J. M. & Malhotra, R. Orbital evolution of planets embedded in a planetesimal disk. Astron. J. 117, 3041–3053 (1999)

    ADS  Article  Google Scholar 

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We are grateful to R. Gomes and M. Holman for acting as referees on this paper. We also thank L. Dones, W. Bottke, J.-M. Petit, and K. Tsiganis for help with an early version of the text and M. Duncan and R. Gomes for discussions. H.F.L. is grateful for funding from NASA. We thank the CNRS and NSF for encouraging friendly relationships between the US and France.

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Correspondence to Harold F. Levison.

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Levison, H., Morbidelli, A. The formation of the Kuiper belt by the outward transport of bodies during Neptune's migration. Nature 426, 419–421 (2003).

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