The Plurality of Worlds

by William Whewell

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.[9] 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.

FOOTNOTES:

[1] Other speculators also have regarded Saturn's Ring as a ring of cloud or water. See Cosmos, iii. 527 and 553.