# ETI_1 - Extra-Terrestrial Intelligence: How Common Is It?...

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Extra-Terrestrial Intelligence: How Common Is It? Stephen Eikenberry 16 November 2010 AST 2037 1

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The Drake Equation First proposed by Cornell astronomer Frank Drake Actual mathematical formula for estimating number of intelligent civilizations currently in the Galaxy 2
What about number of planets with life? N life = N * f P n e f l So … N * = 3 x 10 11 stars f P = 1/200 fraction of “suitable stars” (an educated guess) n e =1 suitable planets per suitable stars f l = fraction of these planets where life forms 3

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What about number of planets with life? N life = N * f P n e f l N life = 3 x 10 11 stars * 1/200 * 1 * f l = 1.5 billion * f l If f l =1 , then we have 1.5 billion life-bearing planets in the Galaxy If f l =1 in a million, then we have 1500 life-bearing planets in the Galaxy If f l =1 in a billion, then we are on the ONLY life- bearing planet in the Galaxy 4
Summary from Last Time Rarity/prevalence of life in the Galaxy depends on several factors How many stars are there in the Galaxy? How many of them are suitable for Life? How many of those stars have suitable planets? How many of those suitable planets around suitable stars produce Life? We can calculate an estimate of the number of civilizations in the Galaxy using the Drake Equation A shorter version tells us how many planets have any life at all Current estimates: as high as 1.5 billion, as low as 1 (us!) 5

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The Drake Equation N civ = N * f P n e f l f i f t L civ / L gal N civ = number of current civilizations in the Galaxy N * = 3 x 10 11 stars; f P = 1/200 fraction of “suitable stars”; n e =1 suitable planets per suitable stars f l = fraction of these planets where life forms f i = fraction of life-bearing planets where intelligence evolves f t = fraction of intelligence-bearing planets where technology develops for communication L civ = average lifetime of a civilization L gal = lifetime of Galaxy 6
Intelligence Fraction What fraction of life-bearing planets produce intelligent life? Remember definition of intelligence: capable of

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## This note was uploaded on 07/26/2011 for the course AST 2037 taught by Professor Eikenberry during the Fall '08 term at University of Florida.

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ETI_1 - Extra-Terrestrial Intelligence: How Common Is It?...

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