i 7 2 POPULAR SCIENCE MONTHLY. 



insignificant in comparison with the larger, that the former could 

 become a cool, life-bearing globe, nourished by the beneficent rays of 

 its organic comrade and master. 



Judged by our terrestrial experience, which is all we have to go 

 by, the magnitude of a planet, if it is to bear life resembling that of 

 the earth, is limited by other considerations. Even Jupiter, which, 

 as far as our knowledge extends, represents the extreme limit of great 

 planetary size, may be too large ever to become the abode of living 

 beings of a high organization. The force of gravitation on the sur- 

 face of Jupiter exceeds that on the earth's surface as 2.64 to 1. Con- 

 sidering the effects of this on the weight and motion of bodies, the 

 density of the atmosphere, the laws of pneumatics, etc., it is evident 

 that Jupiter would, to say the very least, be an exceedingly uncom- 

 fortable place of abode for beings resembling ourselves. But Jupi- 

 ter, if it is ever to become a solid, rocky globe like ours, must shrink 

 enormously in volume, since its density is only 0.24 as compared 

 with the earth. Now, the surface gravity of a planet depends on 

 its mass and its radius, being directly as the former and inversely 

 as the square of the latter. But in shrinking Jupiter will lose none 

 of its mass, although its radius will become much smaller. The 

 force of gravity will consequently increase on its surface as the planet 

 gets smaller and more dense. 



The present mean diameter of Jupiter is 86,500 miles, while its 

 mass exceeds that of the earth in the ratio of 316 to 1. Suppose 

 Jupiter shrunk to three quarters of its present diameter, or 64,800 

 miles, then its surface gravity would exceed the earth's nearly five 

 times. With one half its present diameter the surface gravity would 

 become more than ten times that of the earth. On such a planet a 

 man's bones would snap beneath his weight, even granting that he 

 could remain upright at all! It would seem, then, that, unless we 

 are to abandon terrestrial analogies altogether and " go it blind," we 

 must set an upper limit to the magnitude of a habitable planet, and 

 that Jupiter represents such upper limit, if, indeed, he does not 

 transcend it. 



The question then becomes, Can the faint objects seen by Dr. 

 See and his fellow-observers, in the near neighborhood of certain 

 stars, be planets in the sense just described, or are they necessarily 

 far greater in magnitude than the largest planet, in the accepted 

 sense of that word, which can be admitted into the category viz., 

 the planet Jupiter? This resolves itself into another question: 

 At what distance would Jupiter be visible with a powerful telescope, 

 supposing it to receive from a neighboring star an amount of illu- 

 mination not less than that which it gets from the sun? To be 

 sure, we do not know how far away the faint objects described by 



