THE GENESIS OF THE SOLAR SYSTEM The various transformations of astronomical matter which have so far been discussed have given no clue to the origin of the solar system. The sun and planets can hardly represent the remains of independent condensations in a primaeval nebula, their masses being too unequal to admit of any such explanation. For the same reason they can hardly be the remains of a single star which has broken up by rotational fission. The greatest observed inequality of mass in the constituents of binary stars is about four to one, whereas the sun has more than i,000 times the mass of the largest planet (Jupiter) and something like 8,000,000 times the mass of the smallest (Mercury), even when the asteroids, of smaller mass still, are left out of account. Moreover, as Babinet first pointed out, the total rotational momentum in the whole solar system is so slight that it is practically inconceivable that this amount of rotation could have caused it to break up. For these and other reasons there is now fairly general agreement that excess of rotation cannot have caused the birth of the solar system. The stars are so remote from one another that in general the influence of a star even on its nearest neighbour is entirely in finitesimal. The moon on account of its extreme nearness raises tides on the earth which are a few feet in height, this being, roughly speaking, a io,000,000th part of the earth's radius. But the star Proxima Centauri does not raise tides of this height on the surface of the sun, or even of a i ,000,000th or r ,000,000 part of this height ; the average height of tide which Proxima Centauri raises on the sun is about one-5oth of the radius of an electron. If, however, as we have to suppose, the sun has wandered at random among the stars for a period measured in millions and millions of years, it must at times have had nearer neighbours than Proxima Centauri, and we are free to imagine that at times stars must have passed so near to it as to have raised tides of substantial height on its surface.
The Tidal Theory.—According to the tidal theory, which at present seems to hold out most promise of explaining the origin of the solar system, the planets and their satellites owe their existence to an exceptionally close approach between our sun, then a normal single star unaccompanied by planets, and a passing star. The closer the approach of a wandering star, other things being equal, the higher would be the tides raised on the surface of the sun ; but unless the approach was very close indeed, the tides would subside as the star which caused them receded. Jeans, who has studied the question mathematically, has however found that if the passing star came to within a distance of a few radii, generally about three, of the sun, the tides would take the form of two long arms of gas ejected from opposite points of the sun's surface, and these would not necessarily subside as the disturbing star receded. Under the action of gravitational instability, con densations would form in these long gaseous arms, and calculation shows that the masses of these condensations would be of just about the order of magnitude of the observed masses of the planets.
If a tidal cataclysm of this kind can explain the birth of the planets it can obviously, in general terms at least, explain the birth of satellites out of these planets. For immediately upon the birth of any planet, say Jupiter, the original situation recurs but on a miniature scale. Jupiter now plays the part originally played by the sun, while the sun, or possibly the sun and the wandering star in conjunction, plays the tide-raising part originally assigned to the wandering star. Again we can imagine the ejection of tidal filaments of gas, and again the formation of conden sations in these filaments, each condensation ultimately forming a satellite.
So long as the matter is discussed in general terms, it may look as though the process just described might continue endlessly through generation after generation. But actually the question of scale is found again to intervene and fix a limit. Bodies below a certain size cannot be born by condensation, again for the reason that small amounts of gas do not condense at all, but scatter into space like a puff of gas out of a gas-tap. Calculation indicates that the smallest of the known satellites can only have escaped this fate by liquefying or solidifying immediately after birth, and makes it clear that there is no room for a generation junior to the satellites of the planets.
Not all the gas which is ejected at a tidal disruption will im mediately condense into planets and satellites. Some at least must remain scattered in space, forming a resisting medium which will impede the motions of the planets and satellites. The effect of resistance of this kind is to make an orbit assume a circular shape, and in this way the approximate circularity of the orbits, both of the planets and their satellites, is readily accounted for. Ulti mately this stray gas would either fall back into the sun or con dense round the planets or scatter into space, leaving inter planetary space clear of matter.
Jeffreys has calculated that this whole process would require a period of the order of 7,00o million years. As regards order of magnitude, this is in general agreement with Holmes's estimate, made from a study of the composition of radioactive rocks, that 1,5oo million years have elapsed since the solidification of the earth. Quite possibly, the interplanetary medium has not yet entirely disappeared; the particles which reflect the zodiacal light may well be its last surviving vestiges.
While tfie theory just sketched out shows many points of agree ment with observation, it cannot be denied that it also encounters formidable difficulties. Still, compared with other theories which have so far been suggested, it may fairly be said to show the greatest capacity for accounting for observed facts, and to be free, so far as is at present known, from any insuperable objections.
The Earth-moon System.—Whatever hypothesis we hold as to the origin of the solar system, the earth-moon system presents a rather special problem, since the ratio of the masses of the earth and the moon (about 8i to one) is nearer to equality than any other ratio of masses in the system. Mathematical discussion of the tidal theory shows that the more liquid a planet is when a satel lite is born, the greater will be the mass of the satellite in propor tion to that of the planet ; a satellite born out of a wholly liquid planet would have a mass comparable with that of the planet itself. Thus on the tidal theory the abnormal ratio of mass in the earth-moon system can be explained merely by supposing that the earth was rather more liquid at the time of the birth of the moon than the other planets were when their satellites were born.
An alternative suggestion, put forward by G. H. Darwin, is that the natural period of vibration of the earth was in some past epoch exactly equal to the period of its revolution round the sun, so that, as a result of resonance, the sun raised enormous tides in the earth, which ultimately caused it to divide into two parts. Jeffreys, who has studied this hypothesis in some detail, finds that it gives a reasonable explanation of the existence of the earth moon system. G. H. Darwin also studied the separation of the earth and moon under the influence of the forces arising from the lunar tides, and found that these were adequate to account for the present distance between the earth and the moon.
Although these alternative rather special hypotheses are avail able, it remains simplest and most natural to suppose that the moon was born out of the earth in the general cataclysm which tore the planets out of the sun, and the other satellites out of the planets around which they revolve.
Frequency of Genesis of Solar Systems.—The tidal theory of the genesis of the solar system which has just been explained, postulates a very close meeting of two stars in space. A quite simple discussion shows that meetings of the required closeness are necessarily very rare occurrences, and that even in the life time of millions of millions of years which we have found it necessary to allot to the stars, only a very few stars can have experienced such close encounters. A calculation which allows as best we can for all the factors within our knowledge suggests that the proportion can hardly be greater than about one in a million. It follows that very few stars can be surrounded by systems of planets as our sun is, so that the solar system is some what of the nature of a "freak-formation." This brings about a reversion from the view somewhat widely held last century that every star was the centre of a family of planets; it now seems more likely that for every star which is attended by planets, i,000,000 or more aimlessly radiate light and heat into vacant space. The possible abodes of life in the universe are correspondingly restricted, and terrestrial life is seen to form a larger proportion of the total life of the universe than was at one time imagined.
Our sun is a member of a colony of some thousands of millions of stars, the galactic stellar system. We do not know much about the number or arrangement of stars in the outlying parts of this system, but only in the dense central regions are stars at all likely to pass close enough to one another to produce planets. And here calculation shows that under present conditions planetary systems are only likely to be produced at the rate of about one in 6,000 million years. Thus our solar system with its age of only a few thousand million years is very possibly the youngest planetary system in the whole colony. Our terrestrial civilization, with only some 6,000 years of existence behind it, is almost certainly the youngest civilization.