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


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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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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Correspondence to Craig B. Agnor.

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Agnor, C., Hamilton, D. Neptune's capture of its moon Triton in a binary–planet gravitational encounter. Nature 441, 192–194 (2006).

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