Jupiter Could Be Out to Get Us Many astronomers regard Jupiter as Earth’s bodyguard, diligently attracting and smashing foreign bodies that could collide with Earth. But new simulations suggest that Jupiter’s role in the solar system may not be so beneficial. In fact, the gas giant may just have it in for the inner planets, including our own.The traditional theory has been that comets and other debris get caught in Jupiter’s gravity well and neutralized by the immense planet. But that theory has come under fire in the past year, and new simulations indicate that Jupiter’s gravitational field could be doing more harm than good:

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 the simulation, Jupiter was responsible for most of the debris that entered in the inner solar system. And, dinosaur-killing comets aside, such debris may have had an impact on the development of our planet:
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.
Sniping at Jupiter [via Science Not Fiction ]