Re: Is it not reasonable to assume that many planets existed

First of all, we do not know, as a matter of
established fact, whether life exists anywhere
but here on Earth.  However, it is (in my opinion) 
a reasonable guess that life is widespread in the
universe.  If this hypothesis is correct, then
it would follow that life-bearing worlds have
been destroyed by astrophysical processes, such as
the "death" of their parent suns.  We do know
that our sun is "middle aged":  it was born about
4.5 billion years ago and is about half-way through
it's lifetime.  Our galaxy is (very roughly) 10 billion
year old.