Jupiter may not be the shield against Earth-threatening asteroids and comets that researchers have assumed it was (depicted in this artist's conception). Astronomers simulated the paths through the solar system of rocky objects called centaurs, which give rise to a family of comets passing near Jupiter. The absence of Jupiter would not change the number of comets approaching Earth, although the presence of a Saturn-size planet in its place would increase that number by directing comets our way, the group from England's Open University reported last week at the European Planetary Science Congress in Potsdam.