ON THE ORIGIN OF LIFE 



lar momentum may thus be absorbed by the rotation of double 

 stars or of planets about the star. Actually, the larger stars 

 appear to spin, whereas, the smaller stars show little or no 

 measurable rotation. Although not demonstrated by direct ob- 

 servation, it is very probable that planets do indeed account lor 

 the slow spin of these small, sunlike stars. Since life requires 

 that a planet have an even temperature over a long period of 

 time, it is important to know which stars are most stable. Since 

 luminosity, which is an index of the rate of stellar fuel con- 

 sumption, increases as the 4th power of the star's mass, it is 

 evident that the larger stars would have a shorter equilibrium 

 state. It appears that only the stars belonging to the late F, G, 

 and early K types offer a favorable environment for bioplanets; 

 these smaller stars have both a longer stable state and a greater 

 likelihood of extant planets to absorb excess rotational energy. 

 About 10% of all stars fall in this favorable range. Let us make 

 both an optimistic and a pessimistic estimate, following the 

 original suggestion of Shapley (1957), and see what numbers 

 appear (assuming approximately 10 2 ° stars in the metagalaxy). 



An optimistic estimate is that there are 5 X 10 18 stars with 

 planets of the proper temperature (assuming 10% of all stars 

 have planets, and half of those with planets have one in the 

 right orbit). If 1 in 100 of these suitable planets have evolved 

 life, there are 5 X 10 lt5 bioplanets, and if 1 in 100 bioplanets 

 have evolved intelligent life, there are one-half million billion 

 centers of intelligence. 



Taking a more conservative view and assigning to each of 

 the above probabilities a value of 1 in 1 million (1 in 1 million 

 stars has a planet of the right temperature and size; 1 in a 

 million such planets has life; 1 in a million such bioplanets 

 has intelligent creatures), we still come out with 100 centers of 

 intelligent life. It may be said, therefore, that it is extremely 

 improbable that there is only one center. Indeed, it is likely 

 that the subject of this paper has received a much more detailed 

 treatment previously elsewhere, ". . . seeing," as noted by St. 

 Paul (Heb. 12:1), "we also are compassed about with so great 

 a cloud of witnesses." 



Addendum added in proof: In the years since this lecture was 

 presented a wealth of additional information has been gathered, 



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