LIFE IN SPACE AND TIME I I 



Utterly invisible with the greatest telescopes yet built, 

 or even dreamed of; nor is there any other way at present 

 known to science by which their existence could be certainly 

 detected.* 



We are thus confined perforce to the consideration of 

 the planets of our own solar system, and the number of 

 cases which we have to deal with is cut down from a bilhon 

 to a Httle over a thousand. Most of these, again, drop 

 out, when we consider that the asteroids, which number 

 more than a thousand, are without exception less than 

 500 miles in diameter, and far too small to retain a trace of 

 atmosphere. The same can be said of the 26 satellites of 

 the various planets. Two or three of the largest may be 

 a httle better able to hold an atmosphere than the Moon, 

 but none of them can actually retain one, unless they are 

 too cold to be habitable. 



Only the eight principal planets now remain. One of these 

 is our own home, and out of the contest. Of the rest, the 

 four major planets: Jupiter, Saturn, Uranus and Neptune, 

 appear to be hopeless. All four are of low mean density, 

 and their sohd or hquid cores, if they have such, must be 

 surrounded by atmospheres thousands of miles in depth. 

 Their visible surfaces are composed of clouds, but not of 

 clouds such as those with which we are famihar, for these 

 surfaces are exceedingly cold. 



It is only within recent years that the determination of 

 planetary temperatures by observation has become practi- 

 cable. To attempt it, we must be able to measure the heat 

 which reaches us from the planet; and this is excessively 

 small in amount. With a thermopile composed of wires of 

 brittle alloy, fine as hairs, mounted in a vacuum, with a 

 great reflecting telescope to concentrate the radiation of the 

 planet on such an instrument, and an exceedingly sensitive 

 galvanometer to record the minute currents which are pro- 

 duced in it, the problem has been solved, and extensive 

 radiometric observations have been made by Coblentz and 

 Lampland at the Lowell Observatory, and by Pettit and 



* The partial eclipse of a small star by a very large planet might perhaps 

 produce an observable diminution of light; but this test would not distinguish 

 between a cool planet and a faint companion star, still far too hot to be 

 habitable. 



