242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1961 



be constant for at least this long. Thus we see from the table that 

 only those main-sequence stars at and below spectral type F can sup- 

 port life. Others evolve too fast and do not maintain constant lumi- 

 nosities long enough. 



A second limitation on the development of life on a planet is its star's 

 ability to warm up a large space around it. Stars are j&replaces in the 

 cold and dark of space, each having a region of propitious temperature 

 in which life may develop and survive. It is evident, for example, 

 that the habitable zones of cool stars of spectral type M are much 

 smaller than that of the sun. Therefore, the chance of finding a planet 

 revolving permanently inside the habitable zone of an M star is less 

 than for somewhat hotter stars. However, il/-type dwarfs are far more 

 numerous than any other single spectral type, and the total number of 

 them supporting life may be appreciable. 



Combining the previous two arguments, we conclude that intelligent 

 life has the highest chance of being found in the vicinities of stars of 

 medium temperature, like the sun. A further limitation applies to 

 binary and multiple systems, which constitute about one-third of all 

 stars. A planet associated with a binary may or may not have a stable 

 orbit, and in the latter case could wander out of the habitable zone 

 and destroy life that might have developed earlier. 



As for a life-supporting planet itself, one of its most important 

 qualifications is maintenance of an atmosphere suitable for the chem- 

 ical processes of living beings. An atmosphere makes possible the 

 existence of water or other substances in liquid form on the planet's 

 surface; it is simply inconceivable that living organisms can be 

 maintained without the aid of some substances in liquid form. 



The earth holds its air because its gravitational attraction prevents 

 gas molecules, which are in a state of thermal motion, from escaping. 

 The moon and Mercury are devoid of atmospheres partly because of 

 their smaller surface gravities ; hence, a larger planet is required. 



But it is not advantageous to the emergence of life, especially of 

 a high form, if the planet is too big. Since the most abundant element 

 in the universe is hydrogen, a newly formed planet must have a high 

 percentage of it, particularly in its outer envelope, because of hydro- 

 gen's light weight. In other words, we expect a new planet's atmos- 

 phere to be chemically in a reducing state. As A. I. Oparin has 

 pointed out, life may first appear under reducing conditions, but it 

 seems imlikely that life of a high form would emerge under such a 

 dominantly hydrogen atmosphere. 



My tentative conclusion is based upon the energy metabolism of 

 living beings. In an oxidizing atmosphere, like the earth's, the com- 

 bustion of glucose, 



CeHizOe + 6O2 ^6C02 + 6H2O, 



