ORIGIN OF THE SOLAR SYSTEM 57 



Capture Theory is not, however, stated in so precise a form as 

 the Planetesimal Theory. It is not definitely stated whether 

 all the small particles would revolve in the same direction or 

 not. If they did, then there would be little or no secular effect 

 on the mean distance of a planet. If, however, they moved 

 indifferently in the direct and retrograde senses, then their 

 collective effect would be the same as that of a medium at rest, 

 and the friction encountered by the planets in their motion 

 would cause them to approach the sun. The fact that such a 

 secular effect is stated by See to occur implies that the particles 

 at any point are not on an average supposed to move with the 

 velocity appropriate to a circular orbit at that point, so that 

 the conditions would be such as to ensure that collisions 

 between them would be violent. The small particles are de- 

 scribed by the somewhat vague term of " cosmical dust " ; 

 if this means that they were solid, the Capture Theory, like the 

 Planetesimal Theory, fails on the ground that the collisions 

 between the small particleswould cause the system to degenerate 

 to a gaseous nebula long before any important effect had been 

 produced on the nuclei. If, on the other hand, they were dis- 

 crete molecules, then the system would be a heterogeneous 

 gaseous nebula at the commencement, and this objection does 

 not apply. It is clear, however, that the planets cannot have 

 entered the system from outer space, for then their orbital 

 planes would be inclined to one another at large angles, which 

 the subsequent action of the medium could scarcely affect, 

 whereas actually all the major planets keep very close to the 

 ecliptic. All must, then, be regarded as having always been 

 members of the solar system, however much their orbits may 

 have changed. They are supposed to be derived from the 

 secondary nuclei of a spiral nebula. 



The most important difference between the Planetesimal and 

 Capture theories lies in the history attributed to the satellites. 

 In the former, each satellite is supposed to have always been 

 associated with its present primary, having been near it when 

 originally expelled from the sun. In the Capture Theory, 

 primaries and satellites are both supposed to have initially 

 moved independently round the sun in highly eccentric orbits. 

 If, in the course of its movement, a small body came sufficiently 

 near a large one, and had a sufficiently small relative velocity, 

 then a permanent change would take place in the character of 



