CONDITIONS OF HABITABILITY OF A PLANET. 



93 



its atmosphere is heavily charged with water vapour, and that 

 its climate may not greatly differ on the average from those of 

 certain moist climates within the torrid zone of the earth. 



But the cloudy atmosphere of Venus renders it practically 

 impossible for astronomers to be sure that they have ever seen the 

 permanent markings of its surface, and one great question 

 remains without any certain answer as yet. This is whether 

 Venus, like Mercury, rotates in the same time as it revolves 

 round the sun, or like the earth in about twenty-four hours. 

 In the former case one hemisphere would be perpetually 

 exposed to unendurable heat and the other to unendurable 

 cold, and Venus would be as uninhabitable as Mercury. Yet 

 Schiaparelli and many of our best observers are convinced that 

 this is the condition that actually prevails. Personally I doubt 

 if the evidence is as yet sufficient to warrant us in drawing an 

 assured conclusion, and I am inclined to think that Venus may 

 be rotating in much the same period as the earth. If this be 

 so, then so far as we know, Venus may be a habitable world. 

 Whether it is actually inhabited is a matter entirely beyond 

 our knowledge. 



The outer planets need not detain us. The spectroscope 

 shows us distinctly that Jupiter, Saturn, Uranus, and Neptune 

 all have a considerable amount of native heat, and our observa- 

 tions of Jupiter make it clear that it is still in a condition of 

 constant commotion. Of all these four planets it is improbable 

 that a solid crust has yet begun to form, or water to deposit in 

 the liquid state. They may be better described as small, 

 undeveloped suns than as great, highly developed earths. As 

 for their satellites, though several are larger than the moon, 

 they are all smaller than Mars, and therefore cannot come up 

 to the standard required of a habitable world. 



So in our own system we have found that there is one 

 planet, our earth, that is inhabited, and one other that may 

 perchance be habitable ; the others may with certainty all be 

 ruled out of court. 



We have learnt more. In any system where there are 

 planets revolving round a central sun, the range of distance 

 from that central sun, within which a world must revolve to be 

 habitable, is very restricted, and even within that range of 

 distance the size and density required for that world is very 

 restricted also. The probability, therefore, in any particular case 

 is against a given system containing a habitable world. But 

 systems of two suns or of more, as so many of the stellar 

 systems are, seem quite unfitted to sustain life on their 



