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Neptune's capture of its moon Triton in a binary–planet gravitational encounter

Naturevolume 441, pages192–194 (2006)Cite this article

Abstract

Triton is Neptune's principal satellite and is by far the largest retrograde satellite in the Solar System (its mass is∼40 per cent greater than that of Pluto). Its inclined and circular orbit lies between a group of small inner prograde satellites and a number of exterior irregular satellites with both prograde and retrograde orbits. This unusual configuration has led to the belief that Triton originally orbited the Sun before being captured in orbit around Neptune1,2,3. Existing models4,5,6 for its capture, however, all have significant bottlenecks that make their effectiveness doubtful. Here we report that a three-body gravitational encounter between a binary system (of∼103-kilometre-sized bodies) and Neptune is a far more likely explanation for Triton's capture. Our model predicts that Triton was once a member of a binary with a range of plausible characteristics, including ones similar to the Pluto–Charon pair.

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Figure 1:Exchange capture of Triton.
Figure 2:Outcomes of simulated binary–planet encounters.
Figure 3:Determining capture orbits.
Figure 4:Binaries capable of delivering Triton to Neptune.

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Acknowledgements

We thank E. Asphaug for advice and support and C. McGleam for a critical reading of the manuscript. This work was supported by the National Aeronautics and Space Administration under grants issued through the Planetary Geology and Geophysics, Outer Planets Research, and Origins of Solar Systems programmes.

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Authors and Affiliations

  1. Earth Sciences Department, Center for the Origin, Dynamics and Evolution of Planets, University of California, 1156 High Street, California, 95064, Santa Cruz, USA

    Craig B. Agnor

  2. Department of Astronomy, University of Maryland at College Park, College Park, Maryland, 20742-2421, USA

    Douglas P. Hamilton

Authors
  1. Craig B. Agnor
  2. Douglas P. Hamilton

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Correspondence toCraig B. Agnor.

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Reprints and permissions information is available atnpg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Agnor, C., Hamilton, D. Neptune's capture of its moon Triton in a binary–planet gravitational encounter.Nature441, 192–194 (2006). https://doi.org/10.1038/nature04792

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Editorial Summary

Triton's capture

Unique among the Solar System's large satellites, Triton orbits Neptune in the direction opposite to the planet's spin. Triton's circular orbit is also tilted significantly from Neptune's equator. These facts suggest that the satellite once orbited the Sun before capture by Neptune, but previous models of the capture required improbable events. Craig Agnor and Douglas Hamilton have developed a more natural explanation. In this model, Triton was once a member of a binary that may have been rather like Pluto and its largest moon Charon. The pair strayed too close to Neptune and were torn apart; the companion escaped, but left Triton behind. The cover depicts this encounter. Neptune is orbited by primordial satellites that would not have survived long after Triton's capture. (Graphic based on planetary and satellite maps provided by Steve Albers, Jerry Gardner, James Hastings-Trew, Constantine Thomas and NOAA's ‘Science on a Sphere’ project.)

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