DISCOVERY 



135 



The Origin of the Solar 

 System 



By Harold Jeffreys, M.A., D.Sc. 



Fellow of SI. John's College, Cambridge 



How is it that the earth can keep its atmosphere ? 

 We know that when a person smokes in one comer of 

 a room it is only a few seconds before the smell is 

 noticeable all over it ; and that when a small amount 

 of a gas is let into a chamber from which the air has 

 been exhausted, it expands in a fraction of a second 

 so as to fill it completely. Yet the earth's atmosphere, 

 with a more perfect vacuum outside it than any we 

 can make, manages to persist for thousands and even 

 miUions of years. WTiy docs it not spread out and 

 become lost to us ? The answer illustrates one of the 

 most important principles in modem theories of the 

 origin of the solar system. If a body is thrown up, it 

 falls down again ; but if it was thrown with more than a 

 certain "critical" velocit}', about ii kilometres (7 miles) 

 per second, it would not retum to the earth at all ; it 

 would travel off into space and revolve round the sun. 

 Now a gas consists of a vast number of small particles, 

 called molecules, rushing about in aU directions with 

 different velocities ; their average velocity depends on 

 the constitution of the gas and on its temperature, but 

 is in terrestrial conditions always of the order of a 

 kilometre per second. Individual molecules may, how- 

 ever, attain much greater velocities. If, then, a molecule 

 on the outskirts of the atmosphere attains the critical 

 velocity, it passes off into space, but not otherwise. At 

 the present time this rarely happens ; our atmosphere 

 is being lost so slowly that it will be billions of years 

 before any change is appreciable. 



We know, however, that the earth is very hot inside, 

 and there is very strong geological reason to believe 

 that it is a cooling body, and that a long time ago it 

 was actually liquid at the surface. But that may not 

 have been the start ; before that it may have been 

 largely, perhaps wholly, gaseous. In these circum- 

 stances it must have been much larger in size than it 

 is now, and this would cause the critical velocity to 

 be less. On the other hand, the high temperature would 

 make the velocities of the molecules of the air greater, 

 and it would on both accounts have taken the air a 

 shorter time to be lost. But our atmosphere may 

 have come since then from the inside of the earth, in 

 the form of volcanic gases, and have been gradually 

 altered in composition by Uving organisms. We can 

 go further, however. The iron and sUica that consti- 

 tute most of the earth must have been vapour then, 

 and we can show that these must have been lost had 



the earth had more than three times its present radius ; 

 and then there would have been no earth left. In 

 fact, whatever we suppose to have been the origin of 

 the earth, whether soUd, liquid, or gaseous, it can never 

 in any circumstances have had a specific gravity of 

 less than -J. The larger planets, on account of their 

 greater masses, could hold together when much more 

 widely distended ; but the smaller bodies of the system, 

 such as the asteroids and most of the satelUtes, would 

 have been dispersed for much smaller distensions. In 

 fact we can show that these can never have been gaseous 

 at all, and must therefore have been solid or liquid 

 ever since they had a separate existence. 



The theories of the origin of the planets fall into two 

 classes. In the older theories (those of Laplace, Roche, 

 Fayc, and Lockyer, for example) the formation was 

 supposed to be by some process of gradual condensation 

 from a more or less symmetrical gaseous or quasi- 

 gaseous mass surrounding the sun. The above argu- 

 ment shows that these are untenable. The matter near 

 the earth must have had a density of at least i, other- 

 wise it could never have condensed at all ; and as the 

 sizes of Mercury and Venus are still less, we see that 

 the density nearer the sun must have been greater than 

 this. When we calculate what the mass of the matter 

 within the earth's orbit must have been on tliis basis, 

 we find that it must have been at least a million times 

 what the mass of the sun is now. There is no way in 

 which such a mass of matter can have left the system, 

 and accordingly these hypotheses must be abandoned. 



The condensation must therefore have taken place 

 from a very Umited region, occupying not more than 

 a milUonth of the volume of the solar system. Yet it 

 must have extended from Mercury to Neptune. A 

 mass of this extreme length and thinness could not 

 possibly last more than a few years without breaking 

 up ; it must therefore have been suddenly formed, 

 and broken up into the planets soon afterwards. In 

 a long and detailed investigation Jeans has recently 

 shown that such a filament could have been produced 

 in only one way. When the sun was largely gaseous 

 and much distended, a star considerably more massive 

 than itself must have approached. The sun's envelope 

 became egg-shaped, the sharp end pointing towards 

 the star, and when the star came near enough this end 

 became actually pointed. At this stage the tendency 

 to disruption produced by the star just balanced the 

 sun's tendency to hold together on account of the 

 mutual gravitation of its parts. With a shght further 

 approach the point opened into a gap, and out of it 

 poured the gases of the envelope, slowly at first, more 

 rapidly as the star came nearer, and slowly again as 

 it receded in its hyperbolic or parabolic orbit. The 

 filament shot off was therefore thickest in the middle. 

 As the star passed on it attracted the filament towards 



