292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1963 



surrounded by a nebula of dust and gas. These particles of dust suf- 

 ered collisions with one another and a certain degree of accretion 

 or coagulation occurred. This process continued through a thousand 

 million years or so, the coagulations all the time getting bigger and 

 bigger, with fragmentation occurring as the larger particles collided. 

 Eventually these became powerful accretors of material, and it is 

 possible dynamically to explain with some degree of precision how the 

 planets of various sizes and mass were formed in this manner. 



One uncertainty in the argument concerns the process by which 

 the sun collected the original nebula of gas and dust. There seem 

 to be two possibilities. Interstellar space is full of clouds of dust, 

 and it may be that the sun as it journeyed through space ran into 

 one of these very dense clouds and carried with it tliis large nebula 

 which must at that time have spread over billions of miles representing 

 the extent of planetary orbits. Or it is possible that the event which 

 gives rise to the birth of a star like the sun involves the simultaneous 

 creation of thousands of stars from the primeval cloud of hydrogenous 

 material, and that so much dust and gaseous material remains that 

 the stars themselves are left with a nebula of gas and dust as part 

 of this formation process. 



There are important consequences of these new ideas. On the for- 

 mer ideas that the planets were torn out of the sun, the origin of the 

 solar system was a rare accident; it must have been miique in the 

 entire universe in spite of the vast numbers of stars which existed. 

 In this theory the earth must originally have been extremely hot, 

 and therefore all the biological processes which have since occurred 

 must have been events which took place subsequent to the cooling down 

 of the earth. On the accretion theory the situation is quite different. 

 The formation of planetary systems from the nebulae around stars 

 may be a frequent occurrence in the universe, and our own solar sys- 

 tem can no longer be regarded as unique. Another important corol- 

 lary is that this accretion of planetary systems occurs in a cold state 

 and any prebiotic material wliich exists on the interstellar dust of the 

 nebulae will be carried over to the planets. 



