DATING THE EARTH’S CRUST—-SHAPLEY 143 
of our clues to the mystery. We know too much to permit any naive 
hypothesis to prosper. 
A successful theory must account reasonably well for the follow- 
ing 10 circumstances (the number could be much increased) : 
1. The nine planets move in orbits that are nearly circular. 
2. They carry with them 97 percent of the angular momentum of 
the whole system, and angular momentum cannot be lightly regarded 
in an evolutionary plan. 
3. The planets are well spaced from each other, with the sizes and 
masses tapering off in both directions from Jupiter. 
4. The rotation of the sun, and the orbital revolutions of the nine 
major planets and most of the satellites and asteroids, are in the 
same direction (counterclockwise as seen from Polaris), which is 
also the direction of the axial rotation of most of the planets, if not all. 
5. Because of radiation pressure, dissolution, and dismemberment 
through other causes, the comets are short-lived, relative to the age 
of the earth’s crust; but their abundance and behavior enrolls them 
as a part of the solar family. 
6. The mean density of the earth is 5.5, but of Saturn 0.7 (it would 
float in water !). 
7. A great but faint comet of which the orbit is totally trans-Jovian 
varies unpredictably in brightness by large amounts. 
8. Conspicuous constituents of the solar corona are very highly 
ionized atoms of iron, nickel, and calcium. 
9. The sun’s present isolation in space makes collision or near en- 
counter with another star an improbability of high degree—say, one 
collision in a billion years, to be optimistic. (In very ancient time, as 
noted below, the chances for constructive disaster were probably much 
higher.) 
10. The subsurface temperature and radiation pressure of the sun 
are exceedingly high; the sun is gaseous from surface to center and 
generates energy subatomically in such a manner that the present 
size has not appreciably changed in a billion years or more. 
It would take too much space to comment on the foregoing items, 
or to discuss various hypotheses that have succeeded in satisfactorily 
explaining or harmoniously using a few of them. The most serious 
obstacle to a successful hypothesis is the distribution of the angular 
momentum; the planets have so much, the sun so little. The near- 
circularity of the orbits, and the violently explosive nature of high- 
temperature gas when the pressure is released, are probably the two 
next most difficult properties to handle. 
We generally agree that no hypothesis of the origin of the earth is 
as yet satisfactory. In recording this attitude we have in mind the 
nebular hypothesis associated generally with the names of 
