10 HUMAN BIOLOGY 



corresponds to a thickness of atmosphere so great that, 

 even in the absence of haze, little or no light from without 

 could reach the planet's surface. An atmosphere many 

 hundreds of times as thick as the Earth's would be required 

 to approach this hmit. 



THE HABITABILITY OF KNOWN ASTRONOMICAL BODIES 



We may now pass briefly in review the bodies known 

 to present-day astronomy, examining whether, and to what 

 degree, the conditions for the existence of hfe upon them 

 are met- 



When it is realized that the number of these extra- 

 terrestrial bodies is about a bilHon,* this sounds hke a 

 formidable project. But the overwhelming majority of 

 this vast multitude may be dismissed at once. Almost 

 all of them are stars, self-luminous masses of incandescent 

 gas with temperatures which rise to milhons of degrees in 

 the interior, and, even at the surface, range from more 

 than 20,ooo°c. down to 2000°c. for a few exceptionally cool 

 stars. There can be no thought of hfe here, unless in poetic 

 fancy, such as that which makes the angel in Moody's 

 "Masque of Judgment" retire to ponder on deep problems of 

 theology: "Where in the Sun's core hght and thought are 

 one. 



The nebulae, too (the only other bodies which are visible 

 at interstellar distances) fade from our hst at once. Some, 

 hke that in Andromeda, are vast clouds of stars, so remote 

 that their very hght takes a miOion years or more to reach 

 us. Others, hke that in Orion, are clouds of gas and dust, so 

 rarefied that there is no more matter in a cubic mile of their 

 substance than in a cubic inch of common air. 



Comets, too, such as we know in our own system, are 

 mere "airy nothings" of very low density, and incapable 

 of supporting life. 



Planets alone, as has already been said, can be habitable, 

 and here the limits of our observing powers begin to be 

 felt. The stars, even the nearest of them, are so remote 

 that planets like ours, if revolving about them, would be 



* The common American usage, according to which a billion = lo' = 

 1,000,000,000, requires explicit statement, but hardly an apology. 



