I think I wrote my argument rather poorly.  Let me try again:<p>I could buy that Jupiter in our particular Solar System had the effect of clearing asteroids in the Earth&#x27;s orbit.  However, that doesn&#x27;t automatically mean that Jupiter was essential.  What if, say, Jupiter&#x27;s presence in the early days of the solar system prevented these asteroids from coalescing further, resulting in the presence of the asteroids in the first place?  Or what if it didn&#x27;t matter in the long run anyway?<p>Okay I just pulled that out of thin air, but what I&#x27;m saying is that there&#x27;s too many unknowns to declare we needed Jupiter for life.  The authors cherry-picks arguments that support their cause.  Also bear in mind that this book was written in 2003, when we knew less than a dozen extrasolar planets and, IIRC, basically none looked like our Solar System (because the instruments weren&#x27;t good enough to detect such systems).  Even now, I don&#x27;t think we know enough about planetary systems to confidently say whether Earth-like systems are common or rare.