406 
The Solar System [oh. xyi 
star is only slightly reduced by the process of disruption. A comparison 
with the actual facts of the solar system makes it clear that the parent sun 
must have approximated much more closely to Roche’s model than to the 
incompressible mass. 
Thus the sun must have had considerable condensation of mass, and the 
earth and the other planets must have been formed from its outermost layers 
in the manner investigated in §§ 377 and 378. 
The Birth of Satellites. 
380 . The principal characteristics of the solar system are reproduced with 
great fidelity in the smaller systems formed by Jupiter and Saturn with their 
accompanying families of satellites. Each of these small systems is so exact 
a replica in miniature of the solar system that no suggested origin for the 
main system can be accepted unless it can account equally for the smaller 
planetary systems; any hypothesis which assigned different origins to the 
main system and the sub-systems would be condemned by its own artificiality. 
Immediately after the birth of any planet, say Jupiter, the original 
situation repeats itself in miniature, Jupiter now playing the part originally 
assigned to the sun, while the sun or the wandering star, or possibly both 
together, play the part of the tide-raising disturber. Again we get the 
emitted filament, again the formation of condensations, and again, as the final 
result, a chain of detached masses. Since Jupiter, the sun and the disturbing 
star are all moving originally in the same plane, Jupiter’s satellites, when 
formed, ought also to move in this plane. Not only Jupiter and his satellites 
but all the other planets and their satellites are observed to move approxi 
mately in the same plane, apart from the exceptions occurring on the outer 
edges of the system which have already been noted in § 2 ; this plane must 
then mark the plane of the orbit of the wandering star which was the author 
of the whole disturbance. 
The great disparity in mass between parent and children which prevails 
in the solar system repeats itself in the planetary systems. The sun’s mass is 
1047 times that of his greatest satellite, Jupiter, while Jupiter’s mass is 
12,300 times that of his largest satellite, and the corresponding ratio in the 
system of Saturn is 4150. The nearest approach to equality of mass is found 
in the system of the earth and moon with a mass-ratio of 81 to 1 . This 
suggests that in each case there must have been great condensation of mass 
in the parent body, and that the satellites have been formed by gravitational 
instability as condensations in filaments of comparatively small mass. 
In systems possessing many satellites, namely those of the Sun, Jupiter 
and Saturn, a general tendency may be detected for the masses to increase 
up to a maximum as we pass outwards through the system, and subsequently 
to fall off to a minimum. In the main system, for instance, there is a regular