152 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1924 



looking as though they had fallen together by accident— had it 

 presented this appearance the problem of its origin might reason- 

 ably be dismissed as hopeless. Not only has the principal system of 

 the sun and its planets got clearly marked characteristics, but also 

 these same characteristics reappear in the smaller systems formed 

 by Jupiter and Saturn, each with its family of satellites. Each of 

 these small systems is, to all intents and purposes, a replica in minia- 

 ture of the solar system, so much so that no suggested origin for 

 one system can be regarded as satisfactory unless at the same time 

 it explains the origin of the other two. The principal features com- 

 mon to the three systems are, that the orbits in all three systems are 

 with few exceptions, all in or close to one plane, that these orbits 

 are all described in the same direction, and that the masses of the 

 secondaries, whether planets or satellites, are all small in compari- 

 son with those of the primaries around which they revolve. Thus 

 the sun has a mass equal to 1,047 times that of his greatest planet, 

 Jupiter, while Jupiter's mass is about 11,000 times that of his most 

 massive satellite. The smallest disparity in mass is found in our 

 own Earth-Moon system with a mass ratio of 81 to 1. In systems 

 possessing many satellites (those of the Sun, Jupiter, and Saturn) 

 there is a general tendency for the masses to increase up to a maxi- 

 mum as we pass outward through the system, and then to decrease 

 to a minimum. Thus in the main system there is a regular progres- 

 sion through Mercury, Venus, Earth, Mars to the maximum mass of 

 Jupiter, broken only by the anomalous position of Mars, while on 

 the descending side the progression through Jupiter, Saturn, Uranus, 

 Neptune fails in regularity only through Neptune being some few 

 per cent more massive than Uranus. 



The main line of evolutionary progress has been supposed to be 

 that of a mass of shrinking, rotating matter — first gaseous, then liq- 

 uid, then solid — left to itself in space. Such a system must show 

 one very marked characteristic throughout its whole career, namely, 

 a plane of symmetry. In its earliest stage of all, when the system 

 is a mere chaos of independent molecules, the plane will coincide 

 with what mathematicians describe as the " invariable plane " of 

 the system. Later, when the mass has assumed the regular shape of 

 a rotating nebula, the plane is the equatorial plane of the nebula, the 

 plane in which the arms subsequently appear and in which the stellar 

 condensations start off in their orbits. The symmetry of spiral 

 nebulae about their equatorial planes would of itself suggest strongly 

 that they have developed to their present formations as rotating 

 bodies practically undisturbed by external influences. 



If our solar system had developed out of an undisturbed rotat- 

 ing mass, it, too, ought to exhibit a plane of symmetry. The orbits 

 of nearly all the planets and their satellites do, in actual fact, lie 



