According to a popular model of planetary interactions, Jupiter plays an entirely protective role in our solar system, shielding Earth and the other inner planets from space debris.

The long-accepted model indicates that the gravity and location of the giant body in the outer solar system deflects comets and other planetesimals — rubble left over from the planet-making process -- that might otherwise bombard Earth.

But a new set of more detailed simulations suggests that Jupiter can sometimes act like a sniper instead of a shield, hurling material toward Earth. Using a model of some 40,000 planetesimals, Kevin Grazier of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues found that debris in the outer solar system initially had circular orbits and posed no threat to Earth or the other inner planets early in the history of the solar system. But the researchers showed that, through a series of close gravitational encounters with the outer planets, especially Jupiter, the objects assumed more elongated orbits and were handed down to the inner solar system.

“In our simulations Jupiter was, in fact, responsible for the vast majority of the encounters that kicked outer planet material into the terrestrial planet region,” the team notes in an October 11 poster presented in Ithaca, N.Y., at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences. “Instead of shielding Earth and other terrestrial planets, Jupiter has in fact, been taking ‘pot shots,'” the team concludes.

Not all of the bullets were destructive, Grazier emphasizes. Some of the material that had been delivered to Earth from the outer solar system contained water and other compounds that could have helped life to gain a foothold.

Don Yeomans of JPL says that Jupiter may still offer some protection. The planet’s reputation for being a big brother to Earth is based on the observations that the large planet deflects hits from long-period comets, rather than whether it deflects hits from rocky, planetoid-like objects, he says. “One would expect Jupiter to control the migration of the particles [Grazier] assumes and some would work their way into the inner solar system,” says Yeomans. “Even so, I'd be surprised if Earth took more planetoid-like hits than Jupiter.”

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