This is a classic question in Freshman introductory astronomy and every student is usually asked to supply their own estimates based on what they know about astronomy using the Drake Equation.
N = N* fp ne fl fi fc fL
N*....In the Milky Way, Although the estimated total mass is around 250 billion solar masses, most stars are much less massive than the sun, so the actual number of stars, N*, is probably closer to one trillion.
Fp.....Although recent detections of planets around nearby stars has only turned up massive planets with 'lethal' orbits, the fraction of stars with planets from these surveys is already 40/150 and more sensitive searches will doubtless reveal many more smaller planets around other stars, so fp could be close to 50%.
Ne..... The number of planets or satellites that have conditions within habitable limits for life, from our solar system, could be as high as 4 (Venus, Earth, Mars, Europa) or more.
Fl....the fraction of habitable planets where life actually evolves is a fascinating study of just how tenacious life seems to be. On Earth, we have fossil evidence of bacteria from 3.8 billion years ago, living under conditions far more hostile than today. We know about 'extremophiles' that live almost in boiling water, embedded in solid rock, or entombed in ice. If Mars meteorites really do show traces of fossil life from that world, then the estimate for this factor could be at least 50% in our system alone.
So...from these factors alone, the number of planets or satellites where life could exist in our galaxy could be of the order of one trillion!!!!
At this point, we leave the realm of estimates based on plausible data, and enter a realm where there are no good ways of checking the values of the factors used.
Fi...On Earth, bacteria have been around for 3.8 billion years, but intelligent life only emerged about 10,000 years ago, so this factor is about 1/380000.
Fc...the fraction of intelligent races that learn the technology to communicate. Well, on Earth that fraction is 100%, but if you look at the several thousand different cultures that have come and gone, only a handful had the curiosity to develop science! So this fraction could be around 1/1000...or even smaller!
FL.... The lifetime of a communicating intelligence for our planet is 50 years...but it could be as long as a civilization can sustain its technological base without destroying its biosphere...say a thousand years or more.
At the SETI Page they have a Drake Equation calculator where you can plug in your own estimates for the factors and see what you get!
Copyright (C) 1997 Dr. Sten Odenwald
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