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.)

About a thousand objects are known to orbit there, some of them quite large, including the planet Pluto.

But there's also a body called 2003 UB313 that's even bigger. That body, unofficially known as Xena, measures 1,500 miles (2,400 kilometers) in diameter, compared to Pluto's 1,400 miles (2,300 kilometers) and Triton's 1,700 miles (2,700 kilometers).

(Read "New Planet Discovered in the Solar System?")

About 10 percent of objects in the Kuiper Belt are paired, or binary.

Pluto itself is half of one of these pairs, orbited by the moon Charon, which is one-eighth of Pluto's mass.

Agnor and Hamilton theorize that Triton originated as a member of one of these Kuiper Belt pairs.

As the pair approached Neptune, the scientists suggest, Neptune's gravity yanked Triton and its companion apart. The process slowed Triton enough to become captured, while flinging the companion off into space.

What might have happened to Triton's one-time companion remains a mystery, the scientists say.

"We don't know the exact mass of the escaping object," Agnor said, "but [pairs] like those observed in the Kuiper Belt seem to work [with this model]."

Star Pairs

The new model dodges the pitfalls that have foiled previous theories of Triton's origins.

It has been difficult to explain how the moon was captured by Neptune, scientists say, because Triton would had to have lost a significant amount of speed in order to be drawn into orbit.

One theory has suggested that Triton smacked into one of Neptune's other moons.

The main difficulty with this theory is that the odds of a collision are fairly small, Agnor said.

Another theory proposes that a band of gas extending outward from Neptune slowed Triton down by aerodynamic braking, "just as if you stick your hand out the window of a car," Agnor said.

But a very large amount of gas would be required to create that effect, Agnor notes. And even if such a band of gas existed, it would continue slowing the moon down until Triton's orbit decayed and the moon crashed into Neptune.

"Both of those models require special conditions to work," Agnor said.

Star Evolution

The new theory resembles accepted models of how stars evolve, says Renu Malhotra, a professor of solar system dynamics at the University of Arizona in Tucson.

Stars, she says, are often formed in clusters, which contain a high percentage of binaries.

When the clusters are young, these binary stars are close enough that it is easy for the stars to steal each other's companions.

But nobody has applied this idea to planets and their moons, she adds.

"The stellar binary [model] has not been previously successfully applied in the context of planetary satellites, so this is a novel hypothesis for the origin of Triton," Malhotra said.

Prior to Agnor and Hamilton's new finding, capture-by-collision was the leading explanation for how Neptune snagged its moon. And it's still possible that such a collision was indeed the cause, the scientists say.

But given the number of binary objects now known to be in the Kuiper Belt, Agnor and Hamilton calculate that their scenario is more than a hundred times more likely.

"All you have to do is get close to the planet to disrupt a binary," Agnor said.

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