Neptune wooed Triton with magnetic charm

Stole passing moon from close companion

A pair of US scientists has come up with a plausible explanation as to how Neptune successfully captured its moon Triton - a process which had previously baffled boffins, who couldn't work out how the moon had lost sufficient energy to settle into a nice, comfy orbit rather than continuing on into the infinite yonder.

One previous theory suggested that Triton - which orbits in the opposite direction to Neptune's rotation in tell-tale captured-body style - must have collided with another satellite, the New Scientist notes. However, if this was small, the resulting collision would not have resulted in Triton's imprisonment; if it was big, Triton would have been pulverised in the process.

The answer is, according to Craig Agnor of the University of California, Santa Cruz, and Douglas Hamilton of the University of Maryland in College Park, that Triton was making its way through the solar system with a companion body which acted like a "retro rocket".

The pair's calculations demonstrate that "as Triton and its partner drew close to Neptune, the giant planet's gravity overwhelmed the attraction between the pair with little regard for the size difference between the companions or the precise distance of the encounter".

We take this to mean that, in the interaction between the three bodies, Triton's chum acted as a sort of brake, allowing the former to take up position around Neptune. Having performed this thoughtful service, it was then "effectively cast off" and went on its lonely way.

The theory has found favour with planetary formation expert Richard Nelson of Queen Mary, University of London. He told NS: "The dynamics of three-body encounters are well studied, usually for three objects of similar mass, but this scenario sounds plausible to me." ®

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