30 HUMAN BIOLOGY 



From the known distances which separate the stars, and 

 their rates of motion, it may easily be shown that, on the 

 average, a star should approach another within a distance 

 equal to that which separates the Earth and Sun, only 

 once in 60,000,000,000,000 years, and a much closer approach 

 would probably be necessary to produce a planetary system. 

 This indicates that such encounters can have happened 

 to but a small fraction of the stars. Evidence that encounters 

 at a greater distance have actually happened is furnished 

 by the binary stars. Such a pair of stars moving together 

 through space, and revolving about one another in orbits, 

 must evidently have had a common origin. It is reasonable 

 to suppose that they have been formed by the division of a 

 single mass. In this case, the original orbit must have been 

 nearly circular. The present orbits are highly eccentric; 

 and the only way in which it appears to be possible to 

 account for this is by the effects of encounters. A passing 

 star, attracting the components in different directions and 

 with different intensity, would disturb their motion and 

 convert a circular orbit into an elHpse. To account for the 

 present high eccentricity it must be assumed that on the 

 average a binary star has met with several such encounters. 

 Jeans, to whom this argument is due, has shown that this is 

 possible, provided that the past lives of the stars extend 

 over thousands of billions of years, so that they have lost 

 considerably in mass since their formation. 



Encounters close enough to produce planetary systems 

 should be much less probable than those which have just 

 been discussed; but, even so, they may well have happened 

 to one star in a thousand, or more. In this case the number 

 of planetary systems may run far into the millions. It is 

 not in- every such system that planets would be found 

 which were even potentially habitable; but the number 

 of such bodies should nevertheless be very large. Upon 

 how many of them life would actually be found is not so 

 easy to estimate. But if life exists in two out of three of the 

 possible places in our system, it may well be abundant 

 elsewhere. 



All told, the existing evidence indicates that the number 

 of worlds in which life is actually to be found within the 



