Popular Science Monthly/Volume 15/May 1879/The Origin of Worlds
|THE ORIGIN OF WORLDS.|
IT is from the order of succession in Nature, and not from the everlasting endurance of her works, that we may reasonably expect the reign of perpetual activity in her wide domains. In the animal and the vegetable kingdoms the ravages of decay and death are eternally repaired by the birth of new representatives of life; and the loss which our continents undergo by occasional submergence is compensated by the appearance of new lands above the waters. Even those stupendous catastrophes involving planetary fate do not make an irreparable loss in the vast array of celestial objects. The matter saved from such mighty wrecks will again be available for useful ends; the forces which seem destroyed in the terrific convulsions only assume other forms to participate in new movements and operations, and even the space-pervading medium, while dooming the present worlds to an end in the distant future, yet contributes much to bring others into being, and to perpetuate the events and the wonders of our universe. A clew to the manner in which such important purposes are achieved is to be found on tracing the fate of planets or of satellites introduced into orbits of the smallest size possible; and these inquiries can be conducted with the aid of mathematical principles which are almost wholly unavailable in pursuing the details of the nebular hypothesis.
In treating on the equilibrium figure of the earth supposed to be a homogeneous fluid, Laplace has been much embarrassed on finding that, if the rotation were so rapid as to reduce the length of the day to two hours and twenty-five minutes, stability would cease to be possible, though the equatorial gravity would be only partially neutralized by centrifugal force. In solving analogous problems respecting the form of satellites confined to very small orbits and distorted from a spherical shape, not only by diurnal motion, but by the more potent influence arising from the attraction of a great central orb, similar results may be obtained, but they admit of a more obvious explanation. The accompanying figure will give an idea of the manner in which, in such dangerous ground, a secondary planet would be affected, especially if it were composed of homogeneous and very yielding materials. If the two bodies were as disproportionate in size as Jupiter and his nearest moon, stability would be impossible on the minor one as soon as gravity
at the extremities of its longest diameter was reduced more than fifty per cent, by the disturbances. Of the fatal effects of a further reduction adequate proofs are afforded by three different modes of investigation which I have given in the "Philosophical Magazine" for 1860, 1861, and 1871. Accordingly, on coming into fatal proximity with its primary, such a satellite would not lose its matter in small portions, but would pass away in one great convulsion which would destroy the planetary condition and give birth to a ring.
The insecurity which analysis shows in this case in the mundane structure arises from the circumstance that, when gravity is reduced to less than one half its value along the greatest axis, the pressure along that line can no longer be made to counterpoise the pressure from other directions. This rule will evidently need but slight modifications, when, instead of being so extremely unequal, both bodies have the same relation of size as that subsisting between our earth and moon, or even such as is represented in our diagram. In this case, however, the lesser body would bear a somewhat greater disturbing influence; but its dismemberment, though of a paroxysmal and a very extensive character, would be confined to the side next the primary. On losing a large portion of its mass, the satellite would swing into a wider orbit; its distance from the primary would for a long period be increased by tidal action, and many ages must elapse before they again became near enough to give occasion for a like convulsive rupture. The incorporation of a large celestial body with a greater one around which it previously revolved would thus be effected by a number of paroxysms, and would not be completed before many billions of years.
The intermittent character of these rare events would be very decided, except, perhaps, when the subordinate body were, like a comet, composed of a profusion of exceedingly rarefied gas surrounding a small, dense, central nucleus. Such differences of density as may be naturally expected in the internal and superficial matter of a satellite would tend to give the convulsive dismemberment a somewhat reduced scale, and to make it recur after less remote periods of time. But this influence would be more than counterbalanced if the incorporating body were solid, as the planetary structure would be preserved for a longer time; but, when the rupture took place, the ruin would be more extensive. Indeed, in the cases most likely to occur, the doomed planet would meet its fate in successive stages, of which the number and magnitude may be estimated with tolerable accuracy. If our moon were made to revolve about 4,500 miles beyond our atmosphere, its coalescence with our globe would be inevitable, and it would take place by about six or eight paroxysmal stages extending over a vast immensity of time. Two or three times the number of such terrific convulsions may be expected in the union between Algol and the large planet which causes his variability; and the same estimate will serve for the binary systems or the physically double stars when after long ages they become close enough for the incorporation of the less with the greater.
It is the terrific conflict of matter on such rare and stupendous events, that awakens the power which is mainly concerned in giving birth to worlds. Large primary planets would be called into being if one or both of the celestial objects undergoing these violent stages of combination had the rank of a sun. The vast mass of matter precipitated to the greater body on these occasions would sweep along its equator with furious velocity. But on the subordinate one, especially in its equatorial regions, the more superficial parts would slide over the internal nucleus in an opposite direction, in consequence of the tidal action, which in the new orbit must be powerful enough to produce not merely waves, but even progressive movements at the rate of many leagues a second. From well-established principles and from facts made known by recent experimental researches of Edlund and Zollner, it is evident that immense currents of electricity would circulate around each mighty orb, but in different directions. On the most stupendous scale the two suns, or the sun and its great planetary attendant, would thus acquire magnetism, but have opposite polarities; and, in moving around their common center of gravity, they would exert over a wide domain the peculiar phenomenon which is but feebly manifested by a rotating horseshoe magnet.
Though the calorific effects of the encounters of great spheres have monopolized the attention of modern scientists, many facts show that mechanical action of the most extreme violence is attended with a larger conversion of energy into electricity and magnetism, and that in the case under consideration these forces must be developed on a more gigantic scale than heat and light. On the fall of a meteorite to the sun after a long course through his atmosphere on November 1, 1859, a disturbance occurred in terrestrial magnetism so quick and remarkable as to excite much attention at several stations of Europe and America. Even this circumstance alone would give grounds for a very high estimate of the magnetic agency called into being, if an amount of matter more than a thousand times that contained in our globe were hurled almost horizontally over a solar surface with a velocity of two or three hundred miles a second.
The consequences of the movements of the two great bodies, with the new properties which they assume in these convulsive stages, may be accurately traced by the aid of scientific principles for which Arago furnished a basis in 1825. Observing that, in the neighborhood of copper, water, glass, and other substances, a magnetic needle had its oscillations curtailed in the same manner as if it encountered the resistance of a medium, he endeavored to unravel the mystery by additional experiments, and was finally led to the discovery of magnetism of rotation. The researches which he commenced were continued successfully by Babbage, by Sir John Herschel, and others; it was found that a horseshoe magnet rotating around its axis would impart its circular motion to disks of copper with which it had no connection; but the inquiry was carried still further by Faraday, who proved all the effects on the electrical development attending the movement. Reasoning from what is known of such kinds of action, it is evident that the rapid revolution of the two great magnetized orbs could not sensibly affect the motion of preexisting planets nor even of asteroids in the solar system; but it would alter much the courses and velocities of meteorites and meteoric dust; and it would be likely to make its influence felt in whirling the nebulous matter supplied by comets or separated from the equator of the greater central sun. At that theatre of violence, the matter would be dissociated perhaps into the sub-elements of Lockyer, and it would be quickly spread around, along the equatorial plane; so that the electro-magnetic power would be favored with a good conductor for extending its control to great distances, and its effects can be traced without having recourse to any unwarranted assumptions respecting the passage of electricity through an absolute vacuum or through interstellar space.
The operation of such an agency in the heavens is shown by researches of a different character. M. Gaston Plante, of Paris, has been led by experimental evidence alone to ascribe the form of the spiral nebulæ to electro-magnetic action; as their peculiar features correspond exactly to that which he produced by powerful electric currents under the controlling influence of a magnet. But the influence of the same forces is strongly impressed on the form of another class of nebulous objects. By investigations similar to those of Laplace in regard to the possible extent of the solar atmosphere, it may be proved that a rare gas surrounding a dense nucleus and with a uniform rotation could not be compressed in a greater degree than to show a thickness two thirds of its equatorial dimensions. Yet in many nebulæa with a central condensation the greater diameter is more than four times the less, and this would seem to indicate the operation of some force like dynamic electricity acting along the plane of the equator of these rarefied objects. The evidence on this point will seem stronger when we recollect that observation gives only an inadequate picture of the effects of this cause; as, in consequence of the position in which they are viewed, planetary nebulæ scarcely ever exhibit the full amount of their ellipticity or compression.
Other facts assist in revealing the nature of the forces at work in these celestial curiosities. Judging from peculiarities they present in the spectroscope, Lockyer and Frankland have concluded that several of the nebulæ must possess an exceedingly low temperature. Yet it is difficult to conceive that such cold, rarefied masses could be self-luminous, or that they could be visible to us even when surrounding a central sun, for gases have but a very feeble power of reflecting light. The difficulty, however, may be removed by supposing that the visibility of these nebulæ depends on the passage through them of electricity developed in some dark or bright binary system on the incorporation of the lesser with the greater orb. In this way an explanation may be found for the mysterious and unaccountable variations in the brightness of these objects. From the careful observations of Hind, D' Arrest, and other astronomers, it has been shown that, in a few cases, nebulæ have declined in light so as to become invisible, but reappeared after a time; thus exhibiting changes equally fatal to the ideas that they are congregations of stars or collections of fire-mist gradually cooling and condensing into planetary systems. But the mystery will be removed when we regard their light as dependent on the electro-magnetic action already described; for in its latter stages, especially when the tides on the smaller member of the binary were drawing to a close, there would be occasional interruptions in the production of electricity and in its passage into space.
In ascribing to meteors an important part in the train of events which these widely extended forces are capable of producing, it is not necessary to adopt the extravagant estimates which were made of the numbers of these vagrant bodies in order to support a recently exploded theory in regard to the origin of celestial light. According to some eminent scientists, the amount of meteoric matter which falls to the sun's surface every year would increase his diameter annually about two hundred and forty feet, and it would exceed the mass of Mars. But from their occasional falls to the earth, and from other facts, it may be safely concluded that the number of meteors which become tenants of the solar dominions in the course of one or two millions of years, would afford material enough to form a planet as large as the earth, even if half their numbers could be made to unite into one body, instead of being allowed to rove indiscriminately through the system and to fall to the larger spheres. Now, the arrangement necessary for such a union would arise in our supposed binary system from the movement of the two suns in their magnetized condition around their common center of gravity. The powerful display of electro-magnetism succeeding each stage of dismemberment would gradually bring the majority of all the wandering meteors into the same plane, and give them orbits of a larger size and constantly approaching nearer to a circular form. Though constantly declining, this force must, during many thousand centuries, exert a predominant sway over meteors and comets, collecting them on the verge of the binary system in such numbers and in such a regular array that their aggregation into one body, though long deferred, would be inevitable. A nucleus once formed would increase by appropriating matter from the zone which it traversed, and, though at first much retarded in its growth, it would after many thousand revolutions attain a planetary size. Being largely composed of gaseous matter and therefore very sensitive to the resistance of a space-pervading medium, the newly formed planet would contract its large orbit; and room would be thus made for bringing into being another mundane structure when, after the lapse of millions of centuries, another paroxysmal stage of incorporation awakened electric energy and prepared the way for a new coalition of the vagrant matter of the celestial regions. After numberless ages the recurrence of the dismemberment would give existence to another planetary orb, and increase the mass of the preexisting ones. Accordingly, the verge of a solar system must be considered as the birthplace of all its primary worlds.
It is evidently in this external zone, where solar attraction is most feeble, that we may hope to find the most favorable conditions for the union of small into large masses. In the asteroidal region two spheres of granite, having each a diameter of one hundred miles, could not control the velocity with which they would sweep by one another on meeting if the planes in which they moved differed one degree in their inclinations to the ecliptic. A slight difference in the size or in the shape of their orbits would also be an unsurmountable barrier to their union. If a collision should occur between two asteroids, they would be only shattered into fragments, and a coalescence into one mass would be rendered more hopeless. But on the extreme verge of a solar system the numerous meteors consigned to large circular orbits lying in the same plane would have very nearly the same velocity in contiguous zones, and would be ready for the work of aggregation when their numbers were sufficiently increased by a long-continued electro-magnetic action.
In such an innumerable group of small and light bodies in symmetrical array, a large meteorite or the nucleus of a comet might become the embryo of a future world which may require many thousand years to attain the mass of one of the average asteroids. But its attraction after a time must become powerful enough to clear a large tract of space of matter, and thus to divide into two zones the great ultra-planetary ring of floating matter, while it must gradually make the paths of the small bodies deviate from true circles. From the outer zone it receives the meteors, which are in the perihelia of their orbits, and have their velocity most rapid; but the meteoric bodies from the internal zone unite with the growing mass near the points at which their motion is reduced to the lowest rate. Accordingly, the rotation of the new world must be in the same direction in which the constituents of the great ring were moving, and in which the parent orbs moved around their common center of gravity. The same direction of motion would also be exhibited by meteors which, instead of incorporating at once with the growing world, only described ellipses around it in accordance with the law of gravity.
In this early stage of its existence a world would be able to acquire a large train of meteors revolving permanently around it chiefly in consequence of two circumstances: The rapid increase in the mass and the attraction of the growing planet will make the velocity gained by bodies in approaching it always less than that lost while they are retiring; and orbits, even when slightly hyperbolic, would be changed into ellipses. Besides this, the vast atmosphere of nebulous matter around the new-born sphere would be more effective for the same end, as it would check the velocity of the passing meteors and cause them to revolve around the growing mass long before they incorporate with it. It is in consequence of these meteoric falls, and not the mere process of cooling, that the abundance of cometary and nebulous matter surrounding a young world is brought into a more dense condition. A planetary atmosphere of oxygen and hydrogen would maintain a gaseous form in spite of the refrigerating influence of many ages; but it would be quickly converted into aqueous vapor by the chemical forces awakened on the fall of a meteoric stone, and in the course of time might become liquid or solid as it parted with heat.
If in the immense annular group of bodies two centers of aggregation formed the two incipient worlds, ever increasing, their attractive power would be likely to form a binary system, both moving around the common center of gravity between them. It is when their conjunction takes place near the point where their orbits come nearest together that such a change may be expected. The inner planet having, then, its minimum and the outer its maximum velocity, the movement in the new binary system would be in the same direction as the common orbital motion around the central suns. To such a course of events may be ascribed the origin of the earth and moon, as well as the connection which exists between them; for even tidal action would be sufficient to reduce the eccentricity of the lunar orbit to its present state. If at that early period meteoric and cometary matter were so abundant that both orbs could become twenty times as large and massive, their distance apart would be so much reduced that the moon would long since have incorporated with our globe by a series of paroxysms which would arouse electro-magnetic forces into action and give birth to a family of satellites.
When, however, two embryonic planets, in the contiguous zones of the great ring of meteors, formed a binary system long before attaining their full size, their union would take place like that of greater masses, and be attended with like consequences. It is reasonable to suppose that, in the early stage of its existence on the verge of the solar-system, Mars, like our earth, received a companion having about one or two per cent. of his mass, but confined to a small orbit. This primitive attendant, which was probably over one thousand miles in diameter, subsequently united with Mars by a series of convulsive stages; and, by awakening electric agencies, gave birth to a family of satellites of which Deimos and Phobos alone remain. The career of Jupiter and Saturn was characterized by the same train of changes and events. When they first sprang into existence, in the outer zone of our system, each of these great planets was attended with a large companion which subsequently incorporated with the superior orb by a series of paroxysms, and thus occasioned the birth of a family of minor worlds. Accordingly, in a system of classification based on their modes of origin, neither our moon nor perhaps that of Neptune could be assigned to the same class which includes the satellites of Mars, Jupiter, Saturn, and Uranus.
The very great disproportion between the world-forming power in great and in small binary systems, will appear in a clearer light by considering the violence in both cases attending precipitation from the less to the greater orb. Were our moon placed so near us that it must yield to the rupturing forces, each paroxysmal dismemberment would give to the earth a ring of lunar matter having a transverse section of 30,000 square miles, and forcing its way through the outer terrestrial structure with a velocity of five miles a second. But if the linear dimensions of both bodies were ten times as great, the conflict of the invading mass would be about 100,000 times as violent, and a correspondingly greater amount of energy would be converted into electric, magnetic, and calorific forces. Accordingly, great suns, in passing through their most terrific scenes, call forth a world-making power of the greatest vigor; and will not only give birth to larger spheres, but also send them forth in wider orbits.
But the size and mass which a world attains must depend mainly on the numbers of meteors and comets frequenting the solar dominions while it was in the course of formation. At the birth of Jupiter this vagrant matter was more than usually abundant, and it served to give the planet a predominance over the other members of the solar family. It is very probable that the minute and the rare tenants of space are very numerous in the Milky Way; but this abundance of chaotic material, though calculated to increase the size of worlds, must shorten their term of existence, as the increase which suns obtain in mass and attraction would have the same effect as a resisting medium in abridging the lives of their planets. Events involving the mortality of worlds would thus become more frequent; and it is worthy of remark that it was in or near this part of our universe that most of the temporary stars have sent forth their sudden display of brilliancy. In such a region a planet of large size in closing its career by incorporating with a sun would be attended by an electro-magnetic energy sufficient to give birth to another planetary member of considerable magnitude on the outer zone of the solar system, so that the existence of worlds would not be wholly dependent on the union of double suns.
But even in our own part of the celestial domain there are to be found evident marks of the occurrence of one of those stupendous events to which I have ascribed the appearance of temporary stars, and which are so intimately connected with the birth and death of worlds. On comparing the observations of Carrington and Spoerer with those of Vogel and Young, it appears that for the sun's equatorial zone the time of rotation is scarcely twenty-two days, while it is nearly four weeks for the parallel of fifty degrees. So great and unexpected a difference in the diurnal motion of its parts proves that our central luminary must have at some past time received a large mass, which had a direct motion over his equator, and was finally precipitated to his surface. Whether the incorporating mass was a planet, or the last remains of the great companion which cooperated in giving being to the solar family, the effects deserve attention so far as they show the present working of a power which has been long in a declining condition. The movement of one zone of matter over another having a different velocity of rotation must be a source of solar magnetism, and this force may be therefore regarded as much weaker than it was a million years ago, but much stronger than it will be in very distant future ages.
Yet, even in its reduced state, this magnetic agency is not without control through a wide range of space. If the loss of meteors which become a prey to the attraction of great spheres be replenished by the entrance of new ones into our system, the new visitants from ultra-planetary space would, in consequence of a resisting medium, be found in the greatest numbers along the line of the sun's progressive motion. The arrangement seems, however, to be modified by the sun's magnetism, which, by favoring direct motion in the plane of his equator, gradually leads to the meteoric array which is manifested in the appearance of the zodiacal light. That this light is reflected by innumerable meteors, is an opinion which has long been maintained, and which has been confirmed by late observation; but it is only from the physical consideration which I have presented, that we can account for the permanence of a phenomenon depending on the presence of objects of so minute and perishable a character. To the same cause may be ascribed the direct motion of the comets of short periods of revolution. These effects will give some idea of what might be accomplished by solar magnetism when, as in the cases I have considered, it becomes many million times more powerful than that of our sun, and when it is favored with all the conditions for arranging chaotic matter for a transformation into worlds.