A couple points in defense of the Rare Earth book. The two scientists who
write it make a point that there is no absolute proof for the theory and
that they are biased toward life as it exists on Earth. The other part of
the theory is that life in general is widespread. Proof of that being the
microbes that live along the long cracks that separate the the tectonic
plates under the ocean and all the microbes that live in solid rock. The
Archea I believe they are called. They think that life in the form of
microbes is more widespread in our solar system than previously thought and
throughout the galaxy and universe. One point, however, is that all life
that we know of is based on DNA. Does that mean that life must be based on
DNA? No. But since all life that we know of is based on DNA, that is one of
their assumptions (with a grain of salt).

In other words, they look at all the different conditions that we know about
that make life possible on Earth and based on what is now known in geology
and astronomy and biology, etc., look at how common all the different
factors are (Location in galaxy, large moon in inner planets, planet like
Jupiter to clean up stuff left over from formation of solar system, stable
non-elliptical orbit that keeps Earth in the habitable zone, etc. etc.
etc.). Again, they aren't claiming they have absolute proof that Earth is
rare, but their arguments and discussion are compelling.
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        Tom Hula
        Victor S. Barnes Company
        616.361.7351  x173