Kitabı oku: «The Plurality of Worlds», sayfa 14

CHAPTER X

THEORY OF THE SOLAR SYSTEM

1. We have given our views respecting the various planets which constitute the Solar System;—views established, it would seem, by all that we know, of the laws of heat and moisture, density and attraction, organization and life. We have examined and reasoned upon the cases of the different planets separately. But it may serve to confirm this view, and to establish it in the reader's mind, if we give a description of the system which shall combine and connect the views which we have presented, of the constitution and peculiarities, as to physical circumstances, of each of the planets. It will help us in our speculations, if we can regard the planets not only as a collection, but as a scheme;—if we can give, not an enumeration only, but a theory. Now such a scheme, such a theory, appears to offer itself to us.

2. The planets exterior to Mars, Jupiter, and Saturn especially, as the best known of them, appear, by the best judgment which we can form, to be spheres of water, and of aqueous vapor, combined, it may be, with atmospheric air, in which their cloudy belts float over their deep oceans. Mars seems to have some portion at least of aqueous atmosphere; the earth, we know, has a considerable atmosphere of air, and of vapor; but the Moon, so near to her mistress, has none. On Venus and Mercury, we see nothing of a gaseous or aqueous atmosphere; and they, and Mars, do not differ much in their density from the Earth. Now, does not this look as if the water and the vapor, which belong to the solar system, were driven off into the outer regions of its vast circuit; while the solid masses which are nearest to the focus of heat, are all approximately of the same nature? And if this be so, what is the peculiar physical condition which we are led to ascribe to the Earth? Plainly this: that she is situated just in that region of the system, where the existence of matter, both in a solid, a fluid, and a gaseous condition, is possible. Outside the Earth's orbit, or at least outside Mars and the small Planetoids, there is, in the planets, apparently, no solid matter; or rather, if there be, there is a vast preponderance of watery and vaporous matter. Inside the Earth's orbit, we see, in the planets, no traces of water or vapor, or gas; but solid matter, about the density of terrestrial matter. The Earth, alone, is placed at the border where the conditions of life are combined; ground to stand upon; air to breathe; water to nourish vegetables, and thus, animals; and solid matter to supply the materials for their more solid parts; and with this, a due supply of light and heat, a due energy of the force of weight. All these conditions are, in our conception, requisite for life: that all these conditions meet, elsewhere than in the neighborhood of the Earth's orbit, we see strong reasons to disbelieve. The Earth, then, it would seem, is the abode of life, not because all the globes which revolve round the Sun may be assumed to be the abodes of life; but because the Earth is fitted to be so, by a curious and complex combination of properties and relations, which do not at all apply to the others. That the Earth is inhabited, is not a reason for believing that the other Planets are so, but for believing that they are not so.

3. Can we see any physical reason, for the fact which appears to us so probable, that all the water and vapor of the system is gathered in its outward parts? It would seem that we can. Water and aqueous vapor are driven from the Sun to the outer parts of the solar system, or are allowed to be permanent there only, as they are driven off and retained at a distance by any other source of heat;—to use a homely illustration, as they are driven from wet objects placed near the kitchen-fire: as they are driven from the hot sands of Egypt into the upper air: as they are driven from the tropics to the poles. In this latter case, and generally, in all cases, in which vapor is thus driven from a hotter region, when it comes into a colder, it may again be condensed in water, and fall in rain. So the cold of the air in the temperate zone condenses the aqueous vapors which flow from the tropics; and so, we have our clouds and our showers. And as there is this rainy region, indistinctly defined, between the torrid and the frigid zones on the earth; so is there a region of clouds and rain, of air and water, much more precisely defined, in the solar system, between the central torrid zone and the external frigid zone which surrounds the Sun at a greater distance.

4. The Earth's Orbit is the Temperate Zone of the Solar System. In that Zone only is the play of Hot and Cold, of Moist and Dry, possible. The Torrid Zone of the Earth is not free from moisture; it has its rains, for it has its upper colder atmosphere. But how much hotter are Venus and Mercury than the Torrid Zone? There, no vapors can linger; they are expelled by the fierce solar energy; and there is no cool stratum to catch them and return them. If they were there, they must fly to the outer regions; to the cold abodes of Jupiter and Saturn, if on their way, the Earth did not with cold and airy finger outstretched afar, catch a few drops of their treasures, for the use of plant, and beast, and man. The solid stone only, and the metallic ore which can be fused and solidified with little loss of substance, can bear the continual force of the near solar fire, and be the material of permanent solid planets in that region. But the lava pavement of the Inner Planets bears no superstructure of life; for all life would be scorched away along with water, its first element. On the Earth first, can this superstructure be raised; and there, through we know not what graduation of forms, the waters were made to bring forth abundantly things that had life; plants, and animals nourished by plants, and conspiring with them, to feed on their respective appointed elements, in the air which surrounded them. And so, nourished by the influences of air and water, plants and animals lived and died, and were entombed in the scourings of the land, which the descending streams carried to the bottom of the waters. And then, these beds of dead generations were raised into mountain ranges; perhaps by the yet unextinguished forces of subterraneous fires. And then a new creation of plants and animals succeeded; still living under the fostering influence of the united pair, Air and Water, which never ceased to brood over the World of Life, their Nurseling; and then, perhaps, a new change of the limits of land and water, and a new creation again: till at last, Man was placed upon the Earth; with far higher powers, and far different purposes, from any of the preceding tribes of creatures: and with this, for one of his offices;—that there might be an intelligent being to learn how wonderfully the scheme of creation had been carried on, and to admire, and to worship the Creator.

5. But we have a few more remarks to make on the structure of the Solar System, in this point of view. When we say that the water and vapor of the System were driven to the outer parts, or retained there, by the central heat of the Sun, perhaps it might be supposed to be most simple and natural, that the aqueous vapor, and the water, should assume its place in a distinct circle, or rather a spherical shell, of which the Sun was the centre; thus making an elemental sphere about the centre, such as the ancients imagined in their schemes of the Universe. Nor will we venture to say that such an arrangement of elements might not be; though perhaps it might be shown that no stable equilibrium of the system would be, in this way, mechanically possible. But this at least we may say; that a rotatory motion of all the parts of the universe appears to be a universal law prevalent in it, so far as our observation can reach: and that, by such rotation of the separate masses, the whole is put in a condition which is everywhere one of stable equilibrium. It was, then, agreeable to the general scheme, that the excess of water and vapor, which must necessarily be carried away, or stored up, in the outer regions of the System, should be put into shapes in which it should have a permanent place and form. And thus, it is suitable to the general economy of creation, that this water and vapor should be packed into rotating masses, such as are Jupiter and Saturn, Uranus and Neptune. When once collected in such rotating masses, the attraction of its parts would gather it into spheroidal forms; oblate by the effect of rotation, as Jupiter, or perhaps into annular forms, like the Ring of Saturn;⁵⁷ for such also is a mechanically possible form of equilibrium, for a fluid mass. And these spheroids once formed, the water would form a central nucleus, over which would hang a cover of vapor, raised by the evaporating power of the Sun, and forming clouds, where the rarity of the upper strata of vapor allowed the cold of the external space to act; and these clouds, spun into belts by the rotation of the sphere. And thus, the vapor, which would otherwise have wandered loose about the atmosphere, was neatly wound into balls; which, again, were kept in their due place, by being made to revolve in nearly circular orbits about the Sun.

6. And thus, according to our view, water and gases, clouds and vapors, form mainly the planets in the outer part of the solar system; while masses such as result from the fusion of the most solid materials, lie nearer the sun, and are found principally within the orbit of Jupiter.⁵⁸ To conceive planetary systems as formed by the gradual contraction of a nebular mass, and by the solidification of some of its parts, is a favorite notion of several speculators. If we adopt this notion, we shall, I think, find additional proofs in favor of our view of the system. For, in the first place, we have the zodiacal light, a nebulous appendage to the Sun, as Herschel conceives, extending beyond the orbits of Mercury and Venus. These planets, then, have not yet fully emerged from the atmosphere in which they had their origin:—the mother-light and mother-fire, in which they began to crystallize, as crystals do in their mother-water. Though they are already opaque, they are still immersed in luminous vapor: and bearing such traces of their chaotic state being not yet ended, we need not wonder, if we find no evidence of their having inhabitants, and some evidence to the contrary. They are within a nebular region, which may easily be conceived to be uninhabitable. And where this nebular region, marked by the zodiacal light, terminates, the world of life begins, namely at the Earth.

7. But further, outside this region of the Earth, what do we find in the solar system? Of solid matter, if our views are right, we find nothing but an immense number of small bodies; namely, first, Mars, who, as we have said, is only about one-eighth the earth in mass: the twenty-six small planetoids, (or whatever number may have been discovered when these pages meet the reader's eye,⁵⁹) between Mars and Jupiter; the four satellites of Jupiter; the eight satellites of Saturn; the six (if that be the true number,) satellites of Uranus; and the one satellite of Neptune, already detected. It is very remarkable, that all this array of small bodies begins to be found just outside the Earth's orbit. Supposing, as we have found so much reason to suppose, that Jupiter, and the other exterior planets, are not solid bodies, but masses of water and of vapor; the existence of great solid planetary masses, such as exist in the region of the Earth's orbit, is succeeded externally by the existence of a vast number of smaller bodies. The real quantity of matter in these smaller bodies we cannot in general determine. Perhaps the largest of them, (after Mars,) may be Jupiter's third satellite; which⁶⁰ is reckoned, by Laplace, to have a mass less than 1-10,000th of that of Jupiter himself; and thus, since Jupiter, as we have seen, has a mass 333 times that of the Earth, the satellite would be above 1-30th of the Earth's mass.⁶¹ That none but masses of this size, and many far below this, are found outside of Mars, appears to indicate, that the planet-making powers which were efficacious to this distance from the sun, and which produced the great globe of the Earth, were, beyond this point, feebler; so that they could only give birth to smaller masses; to planetoids, to satellites, and to meteoric stones. Perhaps we may describe this want of energy in the planet-making power, by saying, that at so great a distance from the central fire, there was not heat enough to melt together these smaller fragments into a larger globe;⁶² or rather, when they existed in a nebular, perhaps in a gaseous state, that there was not heat enough to keep them in that state, till the attraction of the parts of all of them had drawn them into one mass, which might afterwards solidify into a single globe. The tendency of nebular matter to separate into distinct portions, which may afterwards be more and more detached from each other, so as to break the nebulous light into patches and specks, appears to be seen in the structure of the resolvable nebulæ, as we have already had occasion to notice. And according to the view we are now taking, we may conceive such patches, by further cooling and concentration, to remain luminous as comets, and perhaps shooting stars; or to become opaque as planets, planetoids, satellites, or meteoric stones. And here we may call to mind what we have already said, that the meteoric stones consist of the same elements as those of the earth, combined by the same laws; and thus appear to bring us a message from the other solid planets, that they also have the same elements and the same chemical forces as the earth has.

8. It has already been supposed, by many astronomers, that shooting stars, and meteoric stones, are bodies of connected nature and origin; and that they are cosmical, not terrestrial bodies;—parts of the solar system, not merely appendages to the earth. It has been conceived, that the luminous masses, which appear as shooting stars, when they are without the sphere of terrestrial influences, may, when they reach our atmosphere, collapse into such solid lumps as have from time to time fallen upon the earth's surface: many of them, with such sudden manifestations of light and heat, as implied some rapid change taking place in their chemical constitution and consistence. If shooting stars are of this nature, then, in those cases in which a great number of them appear in close succession, we have evidence that there is a region in which there is a large collection of matter of a nebulous kind, collected already into small clouds, and ready, by any additional touch of the powers that hover round the earth, to be further consolidated into planetary matter. That the earth's orbit carries her through such regions, in her annual course, we have evidence, in the curious fact, now so repeatedly observed, of showers of shooting stars, seen at particular seasons of every year; especially about the 13th of November, and the 10th of August. This phenomenon has been held, most reasonably, to imply that at those periods of the year, the earth passes through a crowd of such meteor-planets, which form a ring round the sun; and revolving round him, like the other planets, retain their place in the system from year to year.⁶³ It may be that the orbits of these meteor-planets are very elliptical. That they are to a certain extent elliptical, appears to be shown, by our falling in with them only once a year, not every half year, as we should do, if their orbit, being nearly circular, met the earth's orbit in two opposite points. That the shooting stars, thus seen in great numbers when the earth is at certain points of her orbit, are really planetoidal bodies, appears to be further proved by this;—that they all seem to move nearly in the same direction.⁶⁴ They are, each of them, visible for a short time only, (indeed commonly only for a few seconds), while they are nearest the earth; much in the same way in which a comet is visible only for a small portion of its path: and this portion is described in a short time, because they move near the earth. They are so small that a little change of distance removes them beyond our vision.

9. Perhaps these revolving specks of nebulæ are the outriders of the zodiacal light; portions of it, which, being external to the permanently nebulous central mass, have broken into patches, and are seen as stars for the moment that we are near to them. And if this be true, we have to correct, in a certain way, what we have previously said of the zodiacal light;—that no one had thought of resolving it into stars: for it would thus appear, that in its outer region, it resolves itself into stars, visible, though but for a moment, to the naked eye.

10. And thus, all these phenomena concur in making it appear probable, that the Earth is placed in that region of the solar system in which the planet-forming powers are most vigorous and potent;—between the region of permanent nebulous vapor, and the region of mere shreds and specks of planetary matter, such as are the satellites and the planetoidal group. And from these views, finally it follows, that the Earth is really the largest planetary body in the Solar System. The vast globes of Jupiter and Saturn, Uranus and Neptune, which roll far above her, are still only huge masses of cloud and vapor, water and air; which, from their enormous size, are ponderous enough to retain round them a body of small satellites, perhaps, in some degree at least, solid; and which have perhaps a small lump, or a few similar lumps, of planetary matter at the centre of their watery globe. The Earth is really the domestic hearth of this Solar System; adjusted between the hot and fiery haze on one side, the cold and watery vapor on the other. This region only is fit to be a domestic hearth, a seat of habitation; and in this region is placed the largest solid globe of our system; and on this globe, by a series of creative operations, entirely different from any of those which separated the solid from the vaporous, the cold from the hot, the moist from the dry, have been established, in succession, plants, and animals, and man. So that the habitation has been occupied; the domestic hearth has been surrounded by its family; the fitnesses so wonderfully combined have been employed; and the Earth alone, of all the parts of the frame which revolves round the Sun, has become a World.

11. Perhaps it may tend still further to illustrate, and to fix in the reader's mind, the view of the constitution of the solar system here given, if we remark an analogy which exists, in this respect, between the Earth in particular, and the Solar System in general. The earth, like the central parts of the system, is warmed by the sun; and hence, drives off watery vapors into the circumambient space, where they are condensed by the cold. The upper regions of the atmosphere, like the outer regions of the solar system, form the vapors thus raised into clouds, which are really only water in minute drops; while in the solar system, the cold of the outer regions, and the rotation of the masses themselves, maintain the water, and the vapor, in immense spheres. But Jupiter and Saturn may be regarded as, in many respects, immense clouds; the continuous water being collected at their centres, while the more airy and looser parts circulate above. They are the permanent receptacles of the superfluous water and air of the system. What is not wanted on the Earth, is stored up there, and hangs above us, far removed from our atmosphere; but yet, like the clouds in our atmosphere, an example, what glorious objects accumulations of vapor and water, illuminated by the rays of the sun, may become in our eyes.

12. These views are so different from those hitherto generally entertained, and considered as having a sort of religious dignity belonging to them, that we may fear, at first at least, they will appear to many, rash and fanciful, and almost, as we have said, irreverent. On the question of reverence we may hereafter say a few words; but as to the rashness of these views, we would beg the reader, calmly and dispassionately, to consider the very extraordinary number of points in the solar system, hitherto unexplained, which they account for, or, at least reduce into consistency and connection, in a manner which seems wonderful. The Theory, as we may perhaps venture to call it, brings together all these known phenomena;—the great size and small density of the exterior planets;—their belts and streaks;—Saturn's ring;—Jupiter's oblateness;—the great number of satellites of the exterior planets;—the numerous group of planetoid bodies between Jupiter and Mars;—the appearance of definite shapes of land and water on Mars;—the showers of shooting stars which appear at certain periods of the year;—the Zodiacal Light;—the appearance of Venus as different from Mars;—and finally, the material composition of meteoric stones.

13. Perhaps there are other phenomena which more readily find an explanation in this theory, than in any other: for instance, the recent discovery of a dim half-transparent ring, as an appendage to the luminous ring of Saturn, which has hitherto alone been observed. Perhaps this is the ring of vapor which may naturally be expected to accompany the ring of water. It is the annular atmosphere of the aqueous annulus. But, the discovery of this faint ring being so new, and hitherto not fully unfolded, we shall not further press the argument, which, hereafter, perhaps, may be more confidently derived from its existence.

14. There are some other facts in the Solar System, which, we can hardly doubt, must have a bearing upon the views which we have urged; though we cannot yet undertake to explain that bearing fully. Not only do all the planetary bodies of the solar system, as well as the Sun himself, revolve upon their axes; but there is a very curious fact relative to these revolutions, which appears to point out a further connection among them. So far as has yet been ascertained, all those which we, in our theory, regard as solid bodies, Mercury, Venus, the Earth, and Mars, revolve in very nearly the same time: namely, in about twenty-four hours. All those larger masses, on the other hand, which we, in our theory, hold to be watery planets, Jupiter, Saturn, Uranus, revolve, not in a longer time, as would perhaps have been expected, from their greater size, but in a shorter time; in less than half the time; in about ten hours. The near agreement of the times of revolution in each of these two groups, is an extremely curious fact; and cannot fail to lead our thoughts to the probability of some common original cause of these motions. But no such common cause has been suggested, by any speculator on these subjects. If, in this blank, even of hypotheses, one might be admitted, as at least a mode of connecting the facts, we might say, that the compound collection of solid materials, water, and air, of which the solar system consists, and of which our earth alone, perhaps, retains the combination, being, by whatever means, set a spinning round an axis, at the rate of one revolution in 24 hours, the solid masses which were detached from it, not being liable to much contraction, retained their rate of revolution; while the vaporous masses which were detached from the fluid and airy part, contracting much, when they came into a colder region, increased their rate of revolution on account of their contraction. That such an acceleration of the rate of revolution would be the result of contraction, is known from mechanical principles; and indeed, is evident: for the contraction of a circular ring of such matter into a narrower compass, would not diminish the linear velocity of its elements, while it would give them a smaller path to describe in their revolutions. Such an hypothesis would account, therefore, both for the nearly equal times of revolution of all the solid planets, and for the smaller period of rotation, which the larger planets show.

15. In what manner, however, portions are to be detached from such a rotating mass, so as to form solid planets on the one side, and watery planets on the other, and how these planets, so detached, are to be made to revolve round the Sun, in orbits nearly circular, we have no hypothesis ready to explain. And perhaps we may say, that no satisfactory, or even plausible, hypothesis to explain these facts, has been proposed: for the Nebular Hypothesis, the only one which is likely to be considered as worthy any notice on this subject, is too imperfectly worked out, as yet, to enable us to know, what it will or will not account for. According to that hypothesis, the nebular matter of a system, having originally a rotatory motion, gradually contracts; and separating, at various distances from the centre, forms rings; which again, breaking at some point of their circumference, are, by the mutual attraction of their parts, gathered up into one mass; which, when cooled down, so as to be opaque, becomes a planet; still revolving round the luminous mass which remains at the centre. That such a process, if we suppose the consistency, and other properties, of the nebulous matter to be such as to render it possible, would produce planetary masses revolving round a sun in nearly circular orbits, and rotating about their own axes, seems most likely; though it does not appear that it has been very clearly shown.⁶⁵ But no successful attempt has been made to deduce any laws of the distances from the centre, times of rotation, or other properties of such planets; and therefore, we cannot say that the nebular hypothesis is yet in any degree confirmed.

16. The Theory which we have ventured to propose, of the Solar System, agrees with the Nebular Hypothesis, so far as that hypothesis goes; if we suppose that there is, at the centre of the exterior planets, Jupiter, Saturn, Uranus, and Neptune, a solid nucleus, probably small, of the same nature as the other planets. Such an addition to our theory is, perhaps, on all accounts, probable: for that circumstance would seem to determine, to particular points, the accumulation of water and vapors, to which we hold that those planets owe the greater part of their bulk. Those planets then, Jupiter, Saturn, and the others, are really small solid planets, with enormous oceans and atmospheres. The Nebular Hypothesis, in that case, is that part of our Hypothesis, which relates to the condensation of luminous nebular matter; while we consider, further, the causes which, scorching the inner planets, and driving the vapors to the outer orbs, would make the region of the earth the only habitable part of the system.

17. The belief that other planets, as well as our own, are the seats of habitation of living things, has been entertained, in general, not in consequence of physical reasons, but in spite of physical reasons; and because there were conceived to be other reasons, of another kind, theological or philosophical, for such a belief. It was held that Venus, or that Saturn, was inhabited, not because any one could devise, with any degree of probability, any organized structure which would be suitable to animal existence on the surfaces of those planets; but because it was conceived that the greatness or goodness of the Creator, or His wisdom, or some other of His attributes, would be manifestly imperfect, if these planets were not tenanted by living creatures. The evidences of design, of which we can trace so many, and such striking examples, in our own sphere, the sphere of life, must, it was assumed, exist, in the like form, in every other part of the universe. The disposition to regard the Universe in this point of view, is very general; the disinclination to accept any change in our belief which seems, for a time, to interfere with this view, is very strong; and the attempt to establish the necessity of new views discrepant from these has, in many eyes, an appearance as if it were unfriendly to the best established doctrines of Natural Theology. All these apprehensions will, we trust, be shown, in the sequel, to be utterly unfounded: and in order that any such repugnance to the doctrines here urged, may not linger in the reader's mind, we shall next proceed to contemplate the phenomena of the universe in their bearing upon such speculations.