248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



We can also make, heuristically, a prediction regarding the stars 

 that have a good chance of possessing planets. Years ago Otto Struve 

 pointed out that the rapid rotation of early-type main-sequence stars 

 did not occur in classes later than F5. In other words, the average 

 angular momentum per unit mass of main-sequence stars suffers a 

 conspicuous discontinuity at this spectral subdivision. A reasonable 

 possibility to explain this is that planetary systems are formed aromid 

 stars of later spectral types, the unobservable planets absorbing the 

 excess angular momentmn in each case. 



The available evidence, therefore, suggests that most single stars 

 on the main sequence between F6 and perhaps K5 have a good chance 

 of supporting life of an advanced form on their planets. Only a 

 few percent of all stars fall in this range. Within 16.3 light-years 

 (5 parsecs) of the sun, there are 58 other individual stars, 5 of which 

 are unseen companions. Of 26 single stars in this group, only 2 be- 

 sides the sun fall within our limitations for supporting life : Epsilon 

 Eridani, a K2 dwarf, and Tau Ceti, a G4: dwarf. 



Of course, the actual chance of intelligent life appearing is less 

 than a few percent. Even if the size of a planet revolving within a 

 habitable zone is right, its surface topography might not be. If the 

 entire surface were water covered, for instance, a civilization like ours 

 could not develop. Taking everything into consideration, I venture 

 to state that no more than 1 to 2 percent of stars may have at one 

 time or another supported intelligent life. On this basis, there are 

 within 1,000 light-years a few thousand stars around which life of 

 this nature could appear. 



For us on earth, a most interesting question is whether or not 

 intelligent life exists elsewhere right now. What is the chance of 

 finding extraterrestrial contemporaries? No one dares guess how 

 long our civilization will endure. Granted that man does not destroy 

 himself, he still has to face natural calamities, such as a recurrence 

 of the ice ages. Will man's tendency to overspecialization bring about 

 his downfall? I incline to believe that the lifetime of a technological 

 civilization occupies only a very small fraction of the entire period 

 of biological evolution. If so, two such civilizations in different 

 worlds would scarcely be simultaneous. 



An interstellar journey will not be within our means for a long 

 time to come. At the speed of artificial satellites that we have launched 

 so far, it would take hundreds of thousands of years to cover the 10 

 light-years of distance to Epsilon Eridani or Tau Ceti. Tliis leaves 

 us with only radio communication as a possible means of contacting 

 other intelligent beings. This problem has been treated in detail in 

 an article on Project Ozma, by Frank D. Drake of the National Radio 

 Astronomy Observatory, in Sky and Telescope for January 1960 



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