The Outline of Science 

 THE PLANETS 



LIFE IX OTHER WORLDS? 



It is quite clear that there cannot be life on the stars. Noth- 

 ing solid or even liquid can exist in such furnaces as they are. 

 Life exists only on planets, and even on these its possibilities are 

 limited. Whether all the stars, or how many of them, have plane- 

 tary families like our sun, we cannot positively say. If they have, 

 Mieh planets would be too faint and small to be visible tens of 

 trillions of miles away. Some astronomers think that our sun may 

 be exceptional in having planets, but their reasons are speculative 

 and unconvincing. Probably a large proportion at least of the 

 stars have planets, and we may therefore survey the globes of our 

 own solar system and in a general way extend the results to the 

 rest of the universe. 



In considering the possibility of life as we know it we may at 

 once rule out the most distant planets from the sun, Uranus and 

 Neptune. They are probably intrinsically too hot. We may also 

 pass over the nearest planet to the sun, Mercury. We have reason 

 to believe that it turns on its axis in the same period as it revolves 

 round the sun, and it must therefore always present the same side 

 to the sun. This means that the heat on the sunlit side of Mercury 

 is above boiling-point, while the cold on the other side must be 

 between two and three hundred degrees below freezing-point. 



The Planet Venus 



Tin- planet Venus, the bright globe which is known to all as 

 the morning and evening "star," seems at first sight more promis- 

 ing as regards the possibility of life. It is of nearly the same size 

 as the earth, and it has a good atmosphere, but there are many 

 'oiininrrs who believe that, like Mercury, it always presents 

 the sauie faee to the sun. and it would therefore have the same 

 disad vantage a broiling heat on the sunny side and the cold of 



