Number of habitable planets per star.  This is based on our own solar system where we have not actually found evidence of life outside of our own planet.  So it may be a wild guess and the pessimistic estimate of O.2 may be several large orders of magnitude too optimistic.   Earth's multicellular life may be a freak in the universe, evolving from a unique combination of geologic &amp; solar effect circumstances which are astronomically extremely rare.<br><br>Obviously this study required detailed analysis of current related scientific evidence/data.  However IMHO the conclusions are over-simplistic based on our lack of knowledge which must increase the incidence of variables.  So overall it seems to me to be perhaps unrealistic and tempered more by wishful thinking based on scant  evidence.    Will a crab in Alaska, and all its progeny over eons, ever know that there are kangaroos in Australia?