Neptune Moon Was Once Passing Planetoid, Study Says

Richard A. Lovett
for National Geographic News
May 10, 2006

Neptune's giant moon Triton has long been considered one of the solar system's most peculiar objects.

Not only is it unusually large (40 percent more massive than Pluto), but it orbits Neptune backward, the only large moon to orbit in the opposite direction of its host planet's spin.

Now scientists say they may have solved the mystery of Triton—the giant moon was once part of a pair of planetoids that orbited in the farthest reaches of the solar system, the researchers say.

Astronomers have long speculated Triton's large size and reverse orbit were signs that Triton and Neptune haven't always been paired. Triton appears to be an interloper that was captured by Neptune's gravitational field.

But scientists have had difficulty figuring out how such a capture might have occurred.

In tomorrow's issue of the journal Nature, planetary scientists Craig Agnor of the University of California, Santa Cruz, and Douglas Hamilton of the University of Maryland at College Park propose a new theory.

They suggest that Triton originated as one of a pair of planetlike objects that moved together in the outer reaches of the solar system.

When Triton and its unknown companion passed close to Neptune, the researchers say, the result was a gravitational tug-of-war. Triton's companion lost this struggle and was flung into space, while Triton fell into orbit around the planet.

Other theories have been proposed, but they require unlikely combinations of events that Agnor says are "bottlenecks" that make them improbable.

Recent Discoveries

Agnor and Hamilton based their theory on recent discoveries in the far-flung outer reaches of the solar system known as the Kuiper Belt.

(See Virtual Solar System.)

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