228 SEE— THE EXISTENCE OF PLANETS [April 23, 



whereas in reality they are surrounded by planetary system not 

 unlike that which revolves about our sun. 



6. The spiral nebulae indicate very plainly that the motion of 

 the nebulosity is towards the center ; that it was originally at 

 greater distance, but at length captured and brought in nearer and 

 nearer the center by the action of universal gravitation. 



7. It is, therefore, plain that just as our planets were formed 

 in our nebula at a great distance from the sun and afterwards had 

 their orbits reduced in size and rounded up by moving against a 

 resisting medium ; so also planets revolving and passing gradually 

 into order and stability are developed in the nebulous streams which 

 by condensation have formed the other stars, and this makes pos- 

 sible the formation of planetary systems among the stars generally. 



8. We may, therefore, feel entirely certain that the stars which 

 appear to be single — about four fifths of all the stars — have planets 

 revolving about them. And the other fifth are spectroscopic and 

 visual binaries, the planets in this case being so large as to be 

 visible in our telescopes or producing a relative motion in the line 

 of sight which may be recognized by means of the spectrograph. 



9. The historical difficulty of determining whether there are 

 planets about the fixed stars may, therefore, be definitely overcome 

 by the recognition of the true mode of formation of the planetary 

 system. It is not exceptional, as was formerly believed. So long 

 as we held that the planets were thrown oflf, it was not at all cer- 

 tain that the mechanical conditions would permit such detachments 

 elsewhere in the universe, and our solar system might be held to 

 be nearly if not quite unique. Now, however, all such views be- 

 come inadmissible, and we see that our system is typical of the 

 general order of nature. 



10. Accordingly, although the planets about the fixed stars 

 probably will always be invisible, and too small to be detected by 

 the spectrograph, yet it is possible to be quite sure of their existence 

 from the operation of known mechanical laws ; and from the dem- 

 onstrated mode of formation of the solar system. This is a 

 practical application of the capture theory to the larger problems 

 of the universe, and the result is of general interest to every 



