AND ITS INHABITANTS 13 



HYPOTHESIS OF PLANETS DERIVED FROM A SMALL 

 SECONDARY NEBULA 



Distinctive features of the plane tesimal hypothesis. The 

 planetesimal or spiral nebula hypothesis of Chamberlin. and 

 Moulton postulates the sun already in existence from the in- 

 gathering of a primal nebula. It was at some later stage dis- 

 rupted through the tidal forces produced by the close approach 

 and passage of another star. The result was a secondary 

 nebula, but one essentially unlike the primary. The secondary 

 nebula was developed in a plane and initially possessed a 

 spiral form with the sun at its center. All of its parts moved 

 with freedom and independence in elliptic orbits, a point of 

 difference from the Laplacian hypothesis. The nebula con- 

 tained only a minute fraction of the solar matter, but was 

 endowed by the passing star with a great rotational energy, 

 so that, although so insignificant in mass, the planetary matter 

 dominates enormously over the sun in the moment of momen- 

 tum of the whole system. Thus the planetesimal hypothesis 

 is a bold and frank abandonment of the terms of the original 

 or nebular theory. It is too early as yet to predict what will 

 be the ultimate fate of this hypothesis of a secondary, and, in a 

 measure, an accidental origin of the planets, but, as expounded 

 by its originators, it must be regarded as dynamically more 

 satisfactory than the present form of the hypothesis of primary 

 origin. The essential features will, therefore, be presented as 

 the more probable preliminary steps in the genesis of the earth. 



Forces of tidal disruption. The power of stars to disrupt 

 each other without coming into actual contact, merely through 

 relatively close approach, must be understood, as it is the basis 

 of the planetesimal hypothesis. 



The sun and moon raise terrestrial tides by virtue of the 

 pull of gravity and thus modify that spheroidal form of the 

 earth which is given by its own gravity and the centrifugal 



