"One of the most dramatic discoveries of recent years has
   been the the amount of material - in the clouds of gas and 
   dust between the stars - that is involved in the same kind
   of chemical processes, based on carbon, that are important
   for life on Earth.

   The inference is that the complex carbon chemistry ... was
   a natural development from the only slightly less complex
   chemistry of the clouds of material from which the Solar
   System formed, and that where other Earth-like planets
   exist, complex chemistry may have developed towards life
   in a similar way.

   Leaving aside any consideration of whether life might
   develop on different kinds of planet, such as the giant
   planets, this implies that there could be millions of
   civilizations involving life forms based on carbon 
   chemistry in our Galaxy today.  p. 244

   *

   Some exobiologists, though ... point out that the only
   essential requirement for life to exist is a spource of
   energy ... , in principle it wil be possible for life to
   exist almost anywhere in the Universe ..." p. 142
 
   "Companion to the Cosmos" - John Gribbin 1996 
    ISBN 0-297-81725-6

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