The Quest for Life Beyond the Earth' 



By Carl Sagan 



Staff member, Smithsonian Astrophysical Observatory, and Assistant Professor of 

 Astronomy, Harvard University 



[With 4 plates] 



We are not alone in the universe. Among the countless galaxies, 

 each with billions of stars, there must be many planets on which life 

 is now flourishing. Unfortunately, there is little prospect of travel to 

 these distant worlds — at least for the next century or so — and statisti- 

 cal arguments do not satisfy that amalgam of scientific curiosity and 

 the love of high adventure which motivates the search for the beings 

 of other planets. 



But it may not be necessary to venture beyond our solar system. 

 Tlie possibihty that neighboring planets are inhabited, at least by 

 simple organisms, is a concept that is both very old and very popular. 

 Its immediate appeal, however, should be tempered by the facts. De- 

 spite tlie wide range of studies which have already been performed, we 

 do not know whether the other planets of our solar system are in- 

 habited. The problem often reduces to probability considerations, 

 and to estimates of observational reliability. At convenient places 

 in the following discussion I shall try to pause and give brief expres- 

 sion to alternative interpretations. In almost all cases, an optimistic 

 view can be found which holds that the evidence is strongly sugges- 

 tive of, or, at the worst, not inconsistent with, the existence of extra- 

 terrestrial life ; and a pessimistic view can be found, which holds that 

 the evidence adduced in favor of extraterrestrial life is uncon^^ncing, 

 irrelevant, or has an alternative, nonbiological explanation. I leave 

 it to the reader to pick his own way among the factions. 



Extraterrestrial life and the origin of life are questions intertwined. 

 If it appears relatively easy for life to have emerged in the primitive 

 terrestrial environment, it may follow that the origin of life is a fairly 

 general planetary phenomenon. So let us begin with a discussion of 



1 The A. Calvert Smith prize-winning essay at Harvard University for 1964. Reprinted 

 by permission from Harvard Alumni Bulletin, April 4, 1964. 



297 



