LIFE ON OTHER WORLDS — JEANS 149 



are ruled out largely by unsuitable temperatures. It used to be 

 thought that Mars might have had a temperature more suited to life 

 in some past epoch when the sun's radiation was more energetic than 

 it now is, and that similarly Venus can perhaps look forAvard to a 

 more temperate climate in some future age. But these possibilities 

 hardly accord with modern views of stellar evolution. The sun is 

 now thought to be a comparatively unchanging structure, which has 

 radiated much as now through the greater part of its past life and 

 will continue to do the same until it changes cataclysmically into a 

 minute "white dwarf" star. When this happens there will be a fall 

 of temperature too rapid for life to survive anywhere in the solar 

 system and too great for new life ever to get a foothold. As regards 

 suitability for life, the earth seems permanently to hold a unique 

 position among the bodies surrounding our sun. 



Our sun is, however, only one of myriads of stars in space. Our 

 own galaxy alone contains about 100,000 million stars, and there are 

 perhaps 10,000 million similar galaxies in space. Stars are about 

 as numerous in space as grains of sand in the Sahara. What can we 

 say about the possibilities of life on planets surrounding these other 

 suns? 



We want first to know whether these planets exist. Observational 

 astronomy can tell us nothing ; if every star in the sky were surrounded 

 by a planetary system like that of our sun, no telescope on earth could 

 reveal a single one of these planets. Theory can tell us a little more. 

 While there is some doubt as to the exact manner in which the sun 

 acquired its family of planets, all modern theories are at one in sup- 

 posing that it was the result of the close approach of another star. 

 Other stars in the sky must also experience similar approaches, al- 

 though calculation shows that such events must be excessively rare. 

 Under conditions like those which now prevail in the neighborhood 

 of the sun, a star will experience an approach close enough to generate 

 planets only about once in every million million million years. If we 

 suppose the star to have lived under these conditions for about 

 2,000 million years, only one star in 500 million will have experienced 

 the necessary close encounter, so that at most one star in 500 million 

 will be surrounded by planets. This looks an absurdly minute fraction 

 of the whole, yet when the whole consists of a thousand million million 

 million stars, this minute fraction represents two million million stars. 

 On this caculation, then, two million million stars must already be sur- 

 rounded by planets and a new solar system is born every few hours. 

 The calculation probably needs many adjustments; for instance, con- 

 ditions near our sun are not necessarily typical of conditions through- 

 out space and the conditions of today are probably not typical of con- 

 ditions in past ages. Indeed, on any reasonable view of stellar evolu- 



