FINALLY, a plausible explanation for how Neptune captured its errant moon Triton. It seems that Triton was wandering through space locked in the gravitational embrace of a companion when the pair happened to pass by Neptune. The gravity of the giant planet extricated Triton from its partner, flinging one into deep space and keeping the other as a moon.
About 40 per cent larger than Pluto, Triton moves through its tilted orbit in the opposite direction to Neptune鈥檚 rotation. This is the hallmark of a captured body, rather than one formed in situ, but astronomers have never been able to work out how Neptune managed to capture Triton.
The problem has always been that Triton must have lost energy on its way into orbit round Neptune, but nobody could work out how. Hitting an existing moon would have slowed Triton down, but there were problems with that idea. If the moon was small, Triton would not have been captured, while smashing into a big moon would have shattered Triton too. Then there were the thousand-to-one odds against Triton hitting any moon in the first place.
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鈥淭riton must have lost energy on its way into orbit round Neptune, but nobody could work out how鈥�
Happily, no collision is necessary in the new theory by Craig Agnor of the University of California, Santa Cruz, and Douglas Hamilton of the University of Maryland in College Park because Triton鈥檚 companion carries the excess energy away. 鈥淚t鈥檚 like Triton brings a retro rocket with it,鈥� says Hamilton (Nature, DOI: 10.1038/nature04792).
Their calculations show that as Triton and its partner drew close to Neptune, the giant planet鈥檚 gravity overwhelmed the attraction between the pair with little regard for the size difference between the companions or the precise distance of the encounter. In the resulting interaction, Triton effectively cast off its partner for the planet.
鈥淭he dynamics of three-body encounters are well studied, usually for three objects of similar mass but this scenario sounds plausible to me,鈥� says Richard Nelson, a planetary formation expert at Queen Mary, University of London.
