IN STARRY SKIES

Evening Post, Volume CXII, Issue 109, 4 November 1926, Page 19

IN STARRY SKIES

(By "Omega Centauri.")

BIRTH OF PLANETS

THE TIDAL THEORY

Among English astronomers of the present day the 'favourite theory of the Origin of the Solar System is the Tidal Theory, and Jeans is undoubtedly its ablest exponent. In. a masterly Beries of books and papers, he has gradually developed and expounded it. His.great work on the Kinetic Theory of Gases prepared the way, for there aro striking analogies between the atoms of a gas and the stars of a cosmic system. In 1917 he won the Adams Prize of the University of Cambridge for his essay on "The course of evolution of the configurations posBiblo for a rotating and gravitating fluid mass,", and this essay was enlarged and published in 1918 as "Problems in Cosmogony and Stellar Dynamics." In this work Jeans reviews previdus theories. He shows that those of Kant and Laplace, which contained originally little but pure speculation, admit of mathematical verification' or disproof. He submits the matter, to a rigorous test and concludes that tfrcr outstanding features, and two only, survive. They are the supposition that our system originated out of a nebulous mass, of gas, and the supposition that tho change from the primitive to the present stage has been prodnced mainly by the agency of increasing rotation. Laplace imagined the system to live its Jif c and to undergo its development alone in a vast space, undisturbed arid uninfluenced by its neighbours. Nature, Jeans finds, in spite of its vast store, is not so lavish of its space. Intense disturbances are indeed extremely rare, but eosmical bodies and systems are never immune from tho influence of their neighbours. : See, Arrhenins, Ch*mberlin, and Monlton all recognise the importance of mutual action in eosmical evolution. . The most complete form of the tidal-action theory, before Jeans wrote, was the Planetesimal Hypothesis of Chamberlin and Moulton. Jeans finds that with the addition of tidal disturbance Laplace's theory is amazingly fruitful. In his opinion1 it either reveals, or gives a valuable clue to the origin of every normal formation in the sky with one remarkable exception. The one which it fails to reveal is the very one it was framed to account for, viz., the origin of the solar system. The cause of its failure here is a matter of scale. For a system to follow something.like the Laplacian mode of development, even when aided by disturbance from ; without, it must have a certain minimum mass; and . that minimum ■ enormously exceeds the mass of the entire •olar system. With masses comparable with that of the Galaxy or the Spiral Nebulae, the course of development is nof unlike that sketched in the original nebular hypothesis. Jeans treats al such .yTtems as . being originally. ii the state of vast masses of rotating gas He pictures one of tbeso masses aWning the form of an oblate .spheroid, and then as it rotated faster on account of its condensation, departing from the spheroidal form, developing a sharp equatorial edge ,and becoming lenticular in shape. If such a system contracts further, pome matter must be thrown off from it If the rotating mass wore abspr lately alone in space, ejection would take place continuously all round the equator, and the nucleus would become surrounded by a vast nebulous atmosphere. But no system, however, isolated is quite alone in space, and. Jeans relies on the attraction of anothei-sys-tem or of the_ rest of the universe, to localise emission to two antipodal points on the equatorial sharp edge. The material is then thrown out in spiral arms, and- the mass assumes tho shape of a typical spiral' nebula. Condensations occur in these jets, and Jeans cal.eulatea lhat their masses should become approximately those,of individual stars. The dark matter,-which'" forms a line across a spiral nebula; seen edgewise, is interpreted as ejected matter which has darkened as the result.of cooling. From Van Maanen's estimates of motion within a spiral nebula Jeans reaches the surprising conclusion that1 all the matter, which can be seen in the arms of a spiral has been emitted in the last 200,00Q years. This seems quite incredible. In watching a spiral nebula rotating, sending out its spiral arms, and forming condensations around nuclei in them, Jeans feels that we are actually witnessing the birth of suns similar to our own. But when he traces the development of gaseous masses of the order of that of a single sun, he finds a "fundamental distinction. Gravity is unable to hold together a gaseous body of less than a certain mass. Many bodies 'in the solar system are far too smail ever to have existed in the state of gas. The moon, the satellites of Mars, and the lesser satellites •f Jupiter and Saturn could not retain atmospheres if they wore given to them, and, if they had ever been entirely gaseous themselves they would have been dissipated into space. If a rotating nebula had a mass only equal to that of the sun, material ejected from its equator could never condense into planets or satellites. When a mass such as that divided on account of quickened rotation it would break into two noyVery unequal masses, and thus form a typical double star. The nebular hypothesis unmodified cannot stand. Even in the case of a spiral nebula the action of tidal forces caused by other nebulao is found to bo important In the' case of the formation of -a. solar . system disturbance from without is absolutely essential. -- There is abundant evidence that most stars in the course of their lives have been influenced by their neighbours. The character of the influence depends partly on tho size, but chiefly on the proximity, of tho disturber. If he passes at a great distance tho tides he raises disappear when ne is gone, but if ho comes extremely close disruption occurs. According to the tidal theory this was tho origin of our solar system. Long ago when the sun was dark and cold, filling a space far vaster than;its present dimensions, perhaps having a radius as great as that of the orbit of Neptune, a still larger star passed by. This star is believed to have come Within a distance of less than the sun's diameter from its surface. In "Problems of Cosmogony" Jeans pictures this event as having occurred about 300 million years ago, but in his essay on Cosmogony in "Evolution in the Light of-Modern Knowledge," published this year, he speaks of our sun having shrunk to his present dimensions after millions of millions of years of radiation. As the visitor approached, a stream of gas was ejected towards it, and this became colder by radiation and condensed into liquid near its ends. Then it. broke into detached fragments which formed the nuclei of the planets. One of the liquid ends of the filament formed Mercury, Venus, and the earth, the other end formed Neptune. The portion in the centre of the filament, which remained largely gaseous, broke into the {riant planets Jupiter and Saturn. Mars and Uranus may have been originally partly liquid and partly gaseous, and may have lost a considerable part of their original atmospheres. The attraction of the passing star is believed to have given to all these bodies the orbitnl velocities which prevented them falling back into the sun. Their orbits at first are believed to have been extremely eccentric, and when they passed again vary

close to the sun a similar tidal action is believed to have drawn the satellites from them. The more liquid any planet was at the time the greater would be the mass of the satellite in comparison with tha,t of its parent planet. Jeans considers that it is not difficult to account for the systems of Jupiter and Saturn in this way. He acknowledges that tiose of .Keptune, Uranus, and Mars are more difficult to explain, and that the earth-moon system is the most puzzling of all. It is as exceptional amongst tho planets as the solar system is exceptional or unique amongst the stars. In the Halley Lecture of 1922 Jeans said that systems such as our own must be very rare in the sky. He regarded planets as abnormal because events such as those to which they are duo must be extremely rare. "Indeed," he says, "It is just within the bounds of possibility, although quite, I think, outside the bounds of probability, that our system is unique —that out of two or three thousand million stars which people space, our own may be the only one attended by satellites. To carry this train of thought one step farther, it is just possible, although again quite improbable, that our earth may be the only body in the whole universe which is capable of supporting life." In his later essay on Cosmogony, published in "Evolution," Jeans modifies this statement considerably. Besides greatly extending the time scale, he points out again that a star is far more liable to be broken up when it'is in the earlier and more tenuous stages of its existence than when it is as compact as our sun, and. that the chances of break-up are further increased if the stars were originally far more closely packed than they are now. He says that we may reasonably conjecture, therefore, that planetary systems, although not the normal accompaniment of a sun, must be fairly freely scattered in space. I Jeans states very clearly that the time for arriving at conclusions has not yet come. In spite of the high promise which the tidal theory holds out, he acknowledges that it is too early.to declare that it can finally explain the origin of the system. Its claim to consideration, he maintains, is that, as far as he knows, it provides the only theory of that origin which is' not open to obvious and insuperable objections. We hope to show that there is another theory which is even more powerful in explaining the characteristics of the system and at the same time is less liable than tho tidal theory to encounter insuperable objections.