ORIGIN OF THE SOLAR SYSTEM 53 



were initially highly heterogeneous, with all but -^ of its mass 

 in the central body. 



2. The matter left behind would not form definite rings ; 

 for a gas has no cohesion, and consequently the separation of 

 matter along the equator would be continuous and lead to 

 another gaseous nebula, not rotating like a rigid body. 



3. A ring could not condense into a planet. 



4. According to the latest work of Jeans, viscosity is in- 

 adequate to make a mass of gas as large as a Laplacian nebula 

 rotate like a rigid body. 



5 . No satellite could revolve in a shorter time than it takes its 

 primary to rotate : this condition is violated by Phobos, the 

 inner satellite of Mars, and by the particles constituting the 

 inner edge of Saturn's ring. 



6. All satellites should revolve in the same direction as their 

 primaries rotate : this condition is violated by one satellite of 

 Saturn and two of Jupiter. 



The second, third, and fourth objections seem quite un- 

 answerable at present. The theory of Gravitational Instability, 

 due to Jeans, is an attempt to pass directly from the sym- 

 metrical nebula to an unsymmetrical one with a secondary 

 nucleus, without the ring as an intermediate stage. 



It will be noticed that Laplace's hypothesis implies that all 

 the planets were formerly gaseous, and hence must have been 

 liquid before they became solid. The question of the course of 

 evolution of a gaseous mass initially heterogeneous with several 

 strong secondary condensations has not hitherto been con- 

 sidered ; such a mass would be free from at least the first four 

 of the objections offered to the standard form of Laplace's 

 hypothesis, and its history would serve as a hypothesis inter- 

 mediate between this and the Planetesimal Hypothesis. 



The Planetesimal Hypothesis. — This hypothesis has been 

 formulated by Chamberlin and Moulton x to avoid the serious 

 defects of the Nebular Hypothesis. It really consists of two 

 separate assumptions, either of which could be discarded without 

 necessarily invalidating the other. The first of these involves the 

 close approach of some wandering star to the sun. This would 

 raise two tidal projections at opposite sides of the sun, and if 

 the disturbance was sufficiently violent, streams of matter 



1 T. C. Chamberlin and R. D. Salisbury, Geology, vol. ii. ; F. R. Moulton, 

 Introduction to Astronomy; T. C. Chamberlin, The Origin of tlie Earth. 



