Mystery planet may have saved Earth

Jupiter may have knocked a giant planet out of the early solar system thus saving the Earth from a possible cosmic collision with Mars, U.S. 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 Neptune\'s orbit now known as the Kuiper belt. The impacts also shoved Jupiter and its giant neighbors inward toward the sun with changes in Jupiter\'s 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 didn\'t 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. A collision between Jupiter and either Uranus or Neptune would have explained it, they said, but such a collision would have sent Uranus or Neptune flying off into space, too. The only thing that would make sense, Southwest researchers decided, was if a fifth -- and now long disappeared -- giant planet had existed alongside Jupiter and the others.