THE SMALLEST PARTICLES OF MATTER 27 



Origin of the Solar System and the Age of the Earth 



The Planetesimal Hypothesis of Professors T. C. Chamberlin 

 and F. R. Moulton early in this century advanced the view that 

 some ten or twenty billion years ago the sun passed sufficiently 

 close to another star to raise vast tides and to stimulate greatly the 

 sun's internal activities, so that vast streams of matter were ejected, 

 both toward and away from the visiting star. While most of this 

 matter fell back into the sun, about one-seventh of one per cent 

 of the sun's mass remained swirling about the sun, all in one direc- 

 tion. The larger masses served as nuclei upon which the smaller 

 ones collected into planets and their satellites. (Despite its nine 

 moons, Saturn still has two rings, apparently consisting of unaggre- 

 gated particles.) This hypothesis accounts well for the fact that 

 all the planets and most of their satellites move in the same direc- 

 tion and approximately in the plane of the ecliptic. The few 

 retrograde bodies may represent "captures," e.g., of some cometary 

 body. 



Professor Lyman Spitzer, Jr., 19 has attacked this hypothesis, and 

 holds that material ejected by the sun or by a star would be too 

 hot to condense to form separate bodies, but would dissipate into 

 space. 20 Astrophysicists face conditions not found on our earth. 

 Thus in some stars, for example, the "dark" companion of Sirius, 

 atomic fragments are so closely packed that a single quart has an 

 estimated mass of about 40 tons. On the other hand, atomic 

 nuclei are much denser, as appears from the statement of Professor 

 W. D. Harkins, quoted above. 



As to the probable age of the earth, there is more agreement. 

 According to Professor Louis B. Slichter, 21 geophysicists recognize 

 three major divisions of the earth: (1) a heavy core with a mean 

 density of 10.7, apparently fluid and possibly consisting of a mix- 

 ture of nickel and iron; (2) a thick but much lighter mantle 

 (density, 3.4 to 5.7) overlying the core and composed of ultrabasic 

 rocks, the iron content increasing with depth; (3) the thin crust 

 above the mantle (total thickness about 20 to 60 kilometers), con- 

 sisting of several layers of lighter rocks. "It has long been recog- 

 nized that the radioactive heat being generated in rocks is large 

 enough to be a major influence in the earth's thermal history . . . 

 Radioactive determinations of old crustal rocks indicate that the 

 time which has elapsed since the solidification of the oldest rocks 

 is about H to 2 billion years." 22 



