When a planetary system forms, the chances that a planet capable of supporting life will be formed are high  The chances that a large planet the size of Jupiter or Saturn will be formed, however, are low  Without Jupiter and Saturn, whose gravitational forces have prevented Earth from being frequently struck by large comets, intelligent life would never have arisen on Earth
Since planetary systems are unlikely to contain any large planets, the chances that intelligent life will emerge on a planet are, ttherefore, low
how likely it is that planetary systems containing large planets will also contain planets the size of Earth