Much has been made of the fact that certain solar systems have what astronomers call 'hot Jupiters'. These are planets the size of Jupiter or even bigger. Instead of orbiting very far from their parent stars, as the gas giants in our solar system do, they orbit much closer. Some even orbit closer than the earth does to the sun!
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The downside to this is that the big planets bully any smaller ones that may have existed, and evict them from the solar system. The powerful gravity eddies of a large planet near a star severely destabilize the orbits of smaller planets, sending them into the parent star or off into space. This means these sorts of solar systems are very unlikely to house life, as there is no where for it to live.
Our own solar system is lucky. With Jupiter so far out, it leaves us well enough alone. However, it plays an important role. Jupiter acts as a comet catcher. Stray comets loitering around the solar system are likely to be caught by its gravity and be consumed. This prevents them from coming father in and potentially striking Earth. Thus, the gravity shadow of Jupiter acts as a shield. Obviously this isn't a perfect system, as any dinosaur skeleton can tell you.