PLANETESIMAL HYPOTHESIS 305 



the sun. The gathering of the scattered planetesimals into the 

 knots to form the planets, planetoids, and satellites is assigned to 

 the coming together of these bodies as they pursued their slightly 

 different orbits, not as the result of falling directly together under 

 the control of gravity. 



It is assumed that the planetesimals had rather highly elliptical 

 orbits arranged in disk-like form. Such orbits would be favorable for 

 the meeting and union of the bodies following them. It can be shown 

 mathematically that under such conditions the addition of planet- 

 esimals to the nuclei would give them more and more circular orbits 

 as the nuclei grew, and it is significant that most of the planetoids 

 (asteroids), which presumably have grown little, have the most 

 eccentric orbits, that Mercury and Mars, the smallest of the planets, 

 have more eccentric orbits than the others, while the orbits of the 

 larger planets approach circularity more closely. The photographs 

 of spiral nebulae show large knots with small ones near them, which 

 appear quite capable of evolution into planets and satellites. They 

 also show small scattered knots susceptible of forming planetoids 

 (asteroids). The earth-moon system is assumed to have been 

 derived from companion nuclei of very unequal sizes. 



The knots might have had a rotary motion at the outset, arising 

 from inequalities of projection at the time of their formation; but in 

 part, the rotations of the planets are assigned to the impacts of the 

 planetesimals as they joined the nuclei to form the planets. There 

 would be no fixed relation between the time of rotation of a planet 

 and the time of revolution of its satellites; the period of the latter 

 might be longer or shorter than that of the former. Even if the revolu- 

 tion-period of a satellite-nucleus was originally the same as the rota- 

 tion-period of the planetary-nucleus, the growth of the planet might 

 draw the satellite nearer to itself and shorten the time of its revo- 

 lution. Thus the difficulty of Phobos and of the innermost part of 

 the ring of Saturn is obviated. The mode of accretion assigned 

 might give rise to forward rotation or to retrograde rotation of the 

 planets and satellites; the forward rotation should be the rule and 

 retrograde rotation the exception, as is the case. In a spiral nebula 

 formed in the way assigned, the outer parts of the arms should be 

 composed of lighter materials than the inner parts, and since the 

 planets were formed from these arms, the inner ones should have 

 higher specific gravities than the outer ones, as is the fact. Other 

 peculiarities of the solar system seem to find a fitting explanation 



