ARTICLES 



THEORIES REGARDING THE ORIGIN ! 

 OF THE SOLAR SYSTEM 



BY HAROLD JEFFREYS, M.A., M.Sc. 

 St./ohn's College, Cambridge 



The question of the origin of the solar system is one that has 

 been a source of speculation for over a hundred years ; but, 

 in spite of the attention that has been devoted to it, no really 

 satisfactory answer has yet been obtained. There are at 

 present three principal hypotheses that appear to contain a 

 large element of truth, as measured by the closeness of the 

 approximation of their consequences to the facts of the present 

 state of the system, but none of them is wholly satisfactory. 

 These are the Nebular Hypothesis of Laplace, the Planetesimal 

 Hypothesis of Chamberlin and Moulton, and the Capture Theory 

 of See. Darwin's theory of Tidal Friction is scarcely a distinct 

 hypothesis, but is mentioned separately on account of its appli- 

 cation to all of the others. The main features of these hypo- 

 theses will be outlined in the present paper. 



The Hypothesis of Laplace. — According to Laplace, the solar 

 system formerly consisted of a very much flattened mass of gas, 

 extending beyond the orbit of Neptune, and rotating like a 

 rigid body. In consequence of radiation of energy this slowly 

 contracted, and in so doing gained so much in angular velocity 

 that the centrifugal force at the equator became greater than 

 gravity, and a ring of matter was left behind along the equator. 

 Further contraction would detach a series of rings. These were 

 then expected to break up in such a way that each produced a 

 gaseous planet. This might later evolve in the same way as 

 the original nebula, thus producing satellites. The criticisms 

 of this hypothesis in its original form are very well known, 

 and will only be summarised here. 



i . The angular momentum of the system when the gaseous 

 central body extended to the orbit of any planet can be calcu- 

 lated, and is not nearly sufficient to cause detachment of matter. 

 Poincare showed that this objection could be met if the nebula 



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