289-292] The Rotational Theory 275 



The main objection to this form of the rotational theory has been already 

 stated ( 253) ; for condensation to occur the emission of matter would have 

 to be very rapid. Calculation shews that the mass of the earth must have 

 been ejected in less than three years, and more probably in a-few months*. 

 A further objection is that even if jets of matter were emitted with 

 sufficient rapidity to condense in the gaseous form, they would condense into 

 masses very much greater than the planets of our system we have seen that 

 to obtain masses comparable with Uranus or Neptune the density must be 

 about 10~ 9 which is about 200 times the mean density (5'5 x 10~ 12 ) of the 

 supposed primitive nebula, and 100,000 times the mean density just calculated 

 for its outer regions. On this form of the hypothesis we are again brought 

 to the conclusion that the planets must have started life in the solid or 

 liquid state, and this is open to the objections mentioned in the last section. 



Finally, an objection to any form of rotational theory is that the central 

 mass ought to continue disintegrating until a double star is formed. Our 

 sun has left off disintegrating and has not formed a double star, and the same 

 is true of all the planets. 



291. We may perhaps sum up as follows. Babinet's criterion in itself 

 has not provided conclusive proof against the solar system having been formed 

 by rotation, and could not in any case do so, for the whole criterion becomes 

 inapplicable as soon as we admit the possibility of interaction between our 

 system and external stars. But exact analysis has shewn that the present 

 angular momentum is excessively low for a system which has broken up by 

 rotation, so that after making full allowance for the possibility of this angular 

 momentum having been reduced since the birth of the system, it still seems 

 highly probable that our system was formed in some other way. Combining 

 this with the circumstance that we have been unable to discover any process 

 of rotational fission which would lead to a final formation resembling our 

 solar system in the least degree, it becomes almost impossible to continue to 

 believe that our system is the result of a rotational break-up. We have 

 conjectured that spiral nebulae, star-clusters, binary and then multiple stars 

 are formed by rotation ; these complete the chain of rotational evolution, and 

 there appears to be no room on this chain for systems like our own. 



THE TIDAL THEORY 



292. It being apparently impossible to explain the genesis of our system 

 in terms of the evolution of a single mass rotating by itself in space, it is 

 natural to examine whether it can be explained in terms of the interaction of 

 two masses. This brings us at once to the tidal theory, which has already 

 been investigated dynamically to a considerable extent. 



* Monthly Notices R.A.S. 77 (1917), p. 197. 



182 



