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CHAPTER XVI 
THE SOLAR SYSTEM 
368 . The original aim of cosmogony was to discover the origin of the solar 
system, but the whole history of cosmogony illustrates how nothing fails so 
surely in science as the direct frontal attack. The plan of action in the present 
book has been to study the various transformations which astronomical matter 
must undergo through the action of physical forces, identifying the formations 
predicted by theory with those observed in the sky when possible. In this 
way it has proved possible to trace out the origin and evolution of many 
astronomical objects, including elliptical and spiral nebulae, star clusters of 
various forms, binary and multiple stars and (conjecturally at least) Cepheid 
and long-period variables. But nowhere have we come upon anything bearing 
the least resemblance to the solar system. 
If the sun had been unattended by planets, its origin and evolution would 
have presented no difficulty. It would have been a quite ordinary star, born 
out of a nebula in the ordinary way, but endowed with insufficient rotation 
to carry it on to the later stages of fission into a binary or multiple system; 
it could in fact be supposed to have had precisely the same evolutionary 
career as half of the stars in the sky. In support of the conjecture that the 
sun had stopped short of fission on its evolutionary career we should only 
have had to note the slowness of its present rotation. A simple calculation 
suggests that the sun has even now only a small fraction of the angular 
momentum necessary for fission, and in the earlier stages in which its dimen 
sions were greater than now, the fraction must have been still less. Even if 
we add the angular momenta of all the planets, as we clearly ought if we are 
supposing that these at one time formed part of the sun, the result is still 
the same; the sun can never have had more than a fraction of the angular 
momentum requisite for fission into a binary system. 
Angular Momentum of the Solar System. 
369 . Such a calculation was first made by Babinet in 1861*. Modern 
investigations have shewn the need for many adjustments to his calculation, 
but it is difficult to challenge his result. The sun’s radiation, as we have 
seen (Chap, x) is carrying angular momentum away with it continually, so 
that the sun’s angular momentum is not constant. Further, encounters with 
other stars or systems may change the sun’s angular momentum. But the 
age of the earth is at most some 5000 million years, and in so short a period 
* Comptes Rendus, lii. (1861), p. 481. See also Moulton, Astrophys. Journ. (1900). p. 108.