13(5 



DISCOVERY 



it, with the result that the whole was given a motion 

 of re%-olution around the sun in the plane of the motion 

 of the star. This narrow gaseous body could not, 

 however, remain long unaltered. It can be proved 

 that any such body must in time break up into a row 

 of fragments on account of the mutual attraction of 

 its parts. Thus the planets were formed, and on 

 account of the variation in thickness the largest were 

 at the medium distances from the sun. At the same 

 time cooling would take place, partly owing to increased 

 radiation from the extended surface, and partly because 

 it would tend to expand under the low pressure ; for 

 expansion leads to cooling. (To observe the opposite 

 effect due to compression, pump up a bicycle tyre 

 quickly and see how hot the pump gets.) Thus much 

 of the matter in it would soon liquefy. In this way a 

 number of planets of varj^ing sizes would be formed, 

 the largest in the middle, and all moving in one direc- 

 tion, and nearly in one plane ; the largest would be 

 mostly or wholly gaseous, and the smaller ones liquid. 

 The resemblance to the actual solar system is obvious ; 

 for Jupiter and Saturn, the largest planets, are at 

 medium distances from the sun, and appear to be still 

 partly gaseous, and all the planets do move nearly in 

 the same plane. The smaller ones have of course 

 solidified since. Thus the theory that was adopted, 

 because it was the only one that could account for the 

 existence of the smaller bodies of the system, accounts 

 immediately for several other striking features. But 

 its success does not end there. 



Not all the ejected matter could collect to form the 

 planets. Much of it quickly spread out through the 

 system before it had time to condense ; this became a 

 very rarefied gaseous mass, filling the whole of the 

 system at least as far out as the orbit of Neptune. It 

 was in motion to start with, and each part of it 

 would continue to revolve round the sun as the 

 planets do. 



The primitive planets would move in very eccentric 

 orbits, passing near the sun at one part of their revo- 

 lution, while their furthest distances would be very 

 great. In moving to and from the sun like this they 

 would experience a great resistance from the gas they 

 were travelling through. Hence this motion would 

 gradually be damped out, and the planets would come 

 to move in nearly circular paths, as thej' do now. At 

 the same time the medium itself would be altered. It 

 would always be tending to spread into outer space, 

 just like a planet's atmosphere, and would therefore 

 be gradually lost. This would be accelerated by the 

 friction of the planets moving through it, which would 

 keep the temperature high. The matter causing the 

 zodiacal light is probably the last remnant of it. The 

 great planets would, by their gravitation, collect large 

 masses of compressed gas about them ; these would 



move with them, thus very much increasing the surface 

 that was being pushed through the medium. This 

 surface would in fact be almost proportional to the 

 square of the mass of the planet, and consequently the 

 larger the planet the more would its orbit be altered 

 by friction. This agrees well with the present eccen- 

 tricities. The four great planets have small eccen- 

 tricities, and the largest eccentricities are those of the 

 smallest planets. Mercury and Mars. The earth and 

 Venus have even smaller eccentricities than Jupiter, 

 but this is only temporary ; it is known that their 

 eccentricities oscillate within wide limits. 



The formation of the satellites is more difficult to 

 trace, not so much on account of any inconsistency with 

 the theory as on account of the variety of methods, 

 all consistent with the theory, that often offer them- 

 selves in particular cases. We shall consider these in 

 turn. 



WTien the planets approached the sun very closely 

 at their nearest points (their " perihelia "), the sun must 

 have raised great tides in them, and this may have 

 caused filaments of matter to be shot out in the same 

 way as the approach of the star caused a filament to 

 be shot out of the sun. In the case of the great planets 

 this caused systems of satellites to be formed, each 

 satellite system resembling the solar system as a whole 

 in that the largest satellites would be at the medium 

 distances, all would revolve in the same direction and 

 approximately in the same plane, and the eccentricities 

 of their orbits would gradually come to be small. This 

 is true of the satellite systems of all the outer planets, 

 except that the outermost sateUite of Saturn and the 

 two outermost of Jupiter revolve in the opposite direc- 

 tion to the others. Also the direction of the revolution 

 should be the same as that of the re\'olution of the 

 planets ; but the four sateUites of Uranus revolve in 

 a plane at right angles to this, and the one sateUite of 

 Neptune revolves in nearly the opposite direction. 

 These are difficulties that await solution. 



Some of the satellites, however, may be as old as 

 the planets. If a small nucleus in the original filament 

 happened to be near a large one when it was formed, 

 it might have continued to revolve round it ever since. 

 For this to happen it must have been a long way off, 

 so that it would not get involved in the dense matter 

 that afterwards condensed into the planet. Now, it 

 appears that if a satellite is at a great distance from 

 its primary, it is least disturbed by the sun's attraction 

 if it moves round the primary in the opposite direction 

 to the usual one. Thus when a satellite in a direct 

 orbit would suffer great perturbations, and perhaps be 

 forced ultimately to leave its primary altogether, one 

 of these " retrograde " satellites might sur\ive. This 

 may be the origin of the retrograde sateUites of Jupiter 

 and Saturn, which are so much more remote from their 



