JUPITER may have knocked a giant planet out of the early solar system thus saving the Earth from a possible cosmic collision with Mars, US astronomers say. At a relatively short 600 million years or so after the solar system formed, Jupiter and the other giant outer planets, Saturn, Uranus and Neptune, were bombarded with chunks of ice and rock, most of which were thrown out into an area beyond Neptunes orbit now known as the Kuiper belt. The impacts also shoved Jupiter and its giant neighbors inward toward the sun with changes in Jupiters orbit that computer models said should have had disastrous effects on the inner planets, possibly even pushing Earth into collision with Mars or Venus. Researchers at the Southwest Research Institute in San Antonio, looking for a reason why that didnt in fact happen, came up with the theory of a fifth giant planet that played a part in restraining the orbital dance of Jupiter, the Houston Chronicle reported Friday. UPI