BIRTH OF PLANETS

Dominion, Volume 31, Issue 162, 5 April 1938, Page 3

BIRTH OF PLANETS

The Sun’s Lost Companion WHIRLING COALESCENCE A reader has asked why the satellites of Neptune and Uranus go round the wrong way, writes E. S. Grew, in the Sunday “Observer.” Planets go round the sun in the same direction that the sun rotates, and rotate as it does. The majority of their twenty-seven satellites follow the example of the harmonious Nine; but before anyone can say why there are exceptions, they have to explain the existence of the rule.

That is equivalent to explaining the origin of the Solar system, which to this day is an unsolved problem. The core of it is the need to explain the speed at which the planets move round the slowly rotating sun. If that could be accounted for, the odd behaviour of a few satellites could take care of itself.

Among the planets Neptune is rather a slowcoach, moving only at three miles a second, but it is so far away and has so long a way to go that it has not yet moved round to the place where it was when first discovered 150 years ago. If we look at it as a stone whirled round at the end of a string, then, multiplying its mass by its immensely long cord, or distance from the sun, we see that its velocity momentum is very great. Faster Tlian Neptune.

So is that of the majority of the planets, most of them moving considerably faster than Neptune. If we add all these travelling momentums together, they work out as very much greater than that of the sun, which, though 700 times their combined mass, merely rotates, slowly, at a speed rather less than half That of Neptune’s onward motion. Thus we have a system of heavenly bodies in which a parent sun has bequeathed’ to its offspring a travelling momentum more, one might say, than it had to give. It seems strange. It is so strange that it killed Laplace’s pretty idea of a spinning nebula, extending as far out as Neptune, and throwing off rings of radiant matter which at last condensed into planets. A system, however it begins, cannot impart to its descendants a travelling momentum—an angular momentum—greater than it possesses. It is as mathematically impossible as the picture of a man pulling himself np by his bootstraps. The system must have been impelled to develop on these lines by a kick from outside. What sort of a kick? When, after a hundred years’ war, the mathematicians finally broke up Laplace’s nebula, one of them, Chamberlain, suggested that the kick was administered by the arrival in the primal sun’s neighbourhood of a dark star. The star, as it approached, drew out of the sun immense tides, like spiral tentacles, and these at length condensed into planets, which, owing to the circumstances of their birth, were invested with high velocities. The Astronomers.

This seemed to offer an explanation of the angular momentum of . the planets, and for the first thirty years of the twentieth century the mathematical astronomers, Sir James Jeans and Dr. Harold Jeffreys, among them, toyed with the new idea.

But it gradually became clear that there were difficulties in the way. It would not be sufficient for the dark horse to gallop near the sun. as near, for example, as the planet Mercury, because at that distance, unless it were incredibly massive, it would nof be able to drag the flaming tides out of ithe sun The new mechanical toy did not break in tlie astronomer's hands, bi,it it wanted mending. Mended it was. first by supposing that tlie raiding star grazed the sun. and then that it. closed with it. But even this would not stand severe examination. and one at least of the chief

mathematical examiners had admitted that the idea of collision would, not Work, when another, Mr. R. A. Lyttelton, came to the rescue. Mr. Lyttelton’s suggestion, made a year and a half ago, was that before the sun acquired a family it was a double star, and that the approaching star knocked one of the companions out, leaving a blazing trail of ruin behind, while one of the three bodies, or perhaps two, galloped off into space. Lyttelton’s melee of three bodies satisfied some of the mathematical necessities of accounting for the condensation of the mass left behind into planets, as well as of the regularity of their distribution and their motions. The Critics. It satisfied a very eminent critic, Professor H. N. Russell, of Princeton. But already gifted objectors have arisen among his own associates. These have urged that the flaming ribbon left behind after the cataclysm would not be long enough for the formation of two planets, the greater part of it having been whirled off into space with the eloping pair in the break-up. Finally, R. G. Gifford, of Wellington, New Zealand, has urged that no filament could be torn off the primal sun unless there was something so near a head-on collision that the tides mingled in a whirling coalescence. This, and the preceding ideas, may prove BjS vulnerable as any other. At present we are reduced to Professor H. N. Russell’s plea that the one thing certain is that the solar system must have had a beginning, and we must go on searching till we And what it Was. But if we can once put the planets in their places, the eccentricities of a few satellites will be a small problem by comparison. The dissentient satellites might have had their motions reversed by early collisions or approaches of two plants before they had settled down in their orbits.