176 MAN'S PLACE IN THE UNIVERSE 



Not only is there a difficulty in ascribing to them per- 

 manent orbits under the more complicated field of gravi- 

 tation, but a cause for the formation of planets seems 

 to be lacking. The star has satisfied its impulse to 

 fission in another manner; it has divided into two nearly 

 equal portions instead of throwing off a succession of 

 tiny fragments. 



The most obvious cause of division is excessive rota- 

 tion. As the gaseous globe contracts it spins fast and 

 faster until a time may come when it can no longer hold 

 together, and some kind of relief must be found. Ac- 

 cording to the nebular hypothesis of Laplace the sun 

 gained relief by throwing off successively rings of matter 

 which have formed the planets. But were it not for 

 this one instance of a planetary system which is known 

 to us, we should have concluded from the thousands of 

 double stars in the sky that the common consequence of 

 excessive rotation is to divide the star into two bodies 

 of equal rank. 



It might still be held that the ejection of a planetary 

 system and the fission into a double star are alternative 

 solutions of the problem arising from excessive rotation, 

 the star taking one course or the other according to 

 circumstances. We know of myriads of double stars 

 and of only one planetary system; but in any case it is 

 beyond our power to detect other planetary systems if 

 they exist. We can only appeal to the results of theo- 

 retical study of rotating masses of gas; the work pre- 

 sents many complications and the results may not be 

 final; but the researches of Sir J. H. Jeans lead to the 

 conclusion that rotational break-up produces a double 

 star and never a system of planets. The solar system is 

 not the typical product of development of a star; it is 

 not even a common variety of development; it is a freak. 



