Postscript: Extraterrestrial Life

Discussions of life on other worlds usually include Drake's Equation:
number of technological civilizations in the galaxy =
rate of star formation * fraction of those with planets * number of habitable planets *

fraction with any life * fraction of those with intelligent life * fraction of those with technology *

lifetime of a technological civilization

As you can see, computing the number of technological civilizations in the galaxy is largely an exercise in guesswork, Most sources choose values for the various factors so that, in effect, the equation becomes
number of technological civilizations in the galaxy ~ lifetime of a technological civilization
Let us ask a more tractable question: how many planets are there in the galaxy which harbor any life at all? We will need to take into account the following:

Since there are approximately 100 billion galaxies in the universe, and about 3/4 of them are spirals, we might expect there to be as many as 3 quintillion planets with life in the universe. Does this number have any basis in fact? We have made a number of educated guesses in this analysis, but it seems fairly obvious that, depending on your definitions, the universe is teeming with extraterrestrial life.
This study indicates that 20% of the stars in our galaxy have habitable planets. If that is true, our analysis is low by a factor of 5.


©2013, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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