LIFE IN SPACE AND TIME 27 



(a quantity whose total amount cannot be altered by any 

 processes occurring within the system) is at present pos- 

 sessed by the planets, which form but 3'foo of the whole 

 mass. There are arguments which approach, if they do 

 not actually constitute, a proof that no such extreme 

 segregation of the angular momentum could be brought 

 about by a gradual process in an isolated system; and it is 

 now generally admitted that here, for once, the catastrophic 

 explanation is to be preferred to the uniformitarian. 



Much the most satisfactory theory of the origin of the 

 planetary system is that which attributes it to a close 

 encounter between the Sun and a passing star whose track 

 almost grazed it. Under the enormous tidal forces huge 

 eruptions of matter would take place from both bodies. Much 

 would fall back on the Sun, or be carried away by the star; but 

 in the rest, set moving sidewise by the attraction of the 

 star as it receded, we may see the raw material of planets, 

 asteroids, satellites, meteors and comets. This hypothesis 

 was first advocated by Chamberlin and Moulton, and 

 has been modified by Jeans and Jeffreys. Into the details 

 of the controversy between the "planetesimal" and "tidal" 

 forms of the theory we need not enter. Many of the details 

 of the origin of our system are still obscure, and some 

 very puzzling; but the general theory of origin by an 

 encounter holds the field. 



During the earlier stages of the system, when much 

 loose matter was still flying about, the orbits of the planets 

 would tend to become nearly circular (as they actually 

 are). An independent estimate of the age of the system 

 may be made from a study of this process. Jeff"reys reaches 

 the figure of 7,000,000,000 years, which is only an indica- 

 tion of the order of magnitude, and theiefore in satisfactory 

 agreement with the radioactive data. Putting all this in a 

 sentence, it appears that the solar system was probably 

 formed by an encounter between the Sun and a passing 

 star, sometime about four billions of years ago. 



How does this compare with the age of the Sun itself, 

 or of the other stars? This question refers, of course, to 

 their age as stars (luminous bodies) and not to that of the 

 matter of which they are composed. 



