Letter

Nature 441, 192-194(11 May 2006) | doi:10.1038/nature04792; Received 29 December 2005; Accepted 31 March 2006

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

Craig B. Agnor1 and Douglas P. Hamilton2

Triton is Neptune's principal satellite and is by far the largest retrograde satellite in the Solar System (its mass is approx40 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 approx103-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.

  1. Earth Sciences Department, Center for the Origin, Dynamics and Evolution of Planets, 1156 High Street, University of California, Santa Cruz, California 95064, USA
  2. Department of Astronomy, University of Maryland at College Park, College Park, Maryland 20742-2421, USA

Correspondence to: Craig B. Agnor1 Correspondence and requests for materials should be addressed to C.B.A. (Email: cagnor@pmc.ucsc.edu).

Received 29 December 2005 |Accepted 31 March 2006

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