For more than a decade many astronomers have thought of Jupiter as a protective big brother for planet Earth. The gas giant's gravitational pull is believed to slingshot incoming Earth-threatening objects out of the Solar System. This has led many to suppose it shielded the young Earth from impacts, helping to support conditions for life. But now, a preliminary study indicates that Earth would have done just as well, if not better — in at least one regard - without Jupiter's help. The results of a study presented today at the European Planetary Science Congress in Potsdam, Germany, indicate that Earth would be struck — by one class of objects at least — at nearly the same rate, regardless of whether Jupiter was there or not. The findings are still tentative, cautions Jonathan Horner, an astronomer at the Open University in Milton Keynes who led the new work. But, he says, Jupiter's role as guardian may have been overstated: "It seems that the idea isn't so clear-cut."The idea of Jupiter as protector was first proposed by planetary scientist George Wetherill in 19941. Wetherill showed that the planet's enormous mass — more than 300 times that of the Earth — is enough to catapult comets that might hit Earth out of the Solar System.

Some have also postulated that Jupiter would thin the crowd of dangerous asteroids and other objects, making Earth a more stable home. Other work has suggested that, in the past, changes in Jupiter's orbit might have actually increased the number of objects on a collision course with earth. Until now, Horner says, little work was done to test either idea. So Horner and colleague Barrie Jones built several versions of the Solar System on the Open University's computer cluster: one with a Jupiter, one without, and several with a gas giant that was either a quarter, half, or three-quarters of Jupiter's mass. The system also contained 100,000 centaurs — large, icy bodies from the Solar System's Kuiper belt, within which Pluto lies.After running their models for 10 million virtual years, Horner and Jones found some striking results. The Earth was about 30% more likely to be hit by a centaur in a Solar System with a life-size Jupiter than it was in a Jupiter-less system.

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If this computer model turns out to be correct, it would significantly increase potential of any other star system to harbor life. Up until now it was suggested that a star system WOULD need a planet size/mass of the Jupiter for there to be long enough period of time without any collisions with this potentially life harboring planet.