156 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



to account for the origin of the solar system. This is one of the most 

 difficult problems in astronomy, which has not yet been completely 

 solved in a satisfactory manner. The only hypothesis which seems 

 to account for the facts is to suppose that a few thousand million years 

 ago another star passed close to the Sun. The passage of the star 

 raised a great tidal protuberance on the Sun, which became greater 

 and greater until a long jet of matter was drawn out. As the stranger 

 star passed on its way, the tidal wave on the Sun subsided but the 

 matter drawn out from the Sun broke up and condensed into planets. 

 The stars are so far apart that such a close encounter of two stars 

 can rarely happen ; calculation suggests that it may occur about once 

 in 5,000 million years. Hence planetary systems are not the rule, 

 but very much the exception, and in our stellar universe there can be 

 but few stars, in addition to the Sun, which have systems of planets 

 attached to them. 



In any planetary system, everything seems to be weighted against 

 the possibility of the existence of life. If the planet is too near its 

 parent sun, it will be too hot for life to exist; if it is too far away, 

 it will be too cold. If it is much smaller than the Earth, it cannot 

 retain any atmosphere. If it is much larger, it will have retained 

 too much atmosphere, for when hydrogen cannot escape, the forma- 

 tion of the poisonous gases, ammonia and marsh gas, appear to be 

 almost inevitable. 



"IT SEEMS PROBABLE THAT THERE ARE OTHER WORLDS WHERE 



LIFE EXISTS" 



Amongst the vast number of stars in any one stellar universe, we 

 should expect to find only a limited number with a family of planets ; 

 and amongst these families of planets there cannot be more than a 

 small proportion where conditions exist that make life possible. On 

 the other hand we must remember the vastness of creation ; there are 

 about 100 million separate universes in the region of space accessible 

 to observation, and we know not how many more beyond. If in each 

 universe there are not more than two or three dozen stars with families 

 of planets, the total number of planetary systems within the relatively 

 small region of space that we can survey is immensely great. If the 

 proportion of planets on which life can exist is not more than one 

 in a million — and our survey of the solar system suggests that this is 

 a considerable underestimate — the total number of planets where con- 

 ditions are suitable for life must be considerable. So it seems probable 

 that there are other worlds where life exists, though that life may be 

 entirely different from any form of life with which we are familiar. 



