The Plurality of Worlds

by William Whewell

27. The numbers of revolutions of the spiral, in the two cases, may not exactly represent the difference of the proportions; but, as I have said, they may serve to show the scale of them; and thus we may say, that if Encke's comet, approaching the centre by 10,000 revolutions, is 100,000 times as dense as the surrounding medium, the elements of the nebula, which reach the centre in a single revolution, are only ten times as dense as the medium through which they have to move.[10]

28. Nor does this result (that the bright element of the nebulæ is so few times denser than the medium in which it moves) offer anything which need surprise us: for, in truth, in a diffused nebula, since we suppose that its parts have mechanical properties, the nebula itself is a resisting medium. The rarer parts, which may very naturally have cooled down in consequence of their rarity, and so, become non-luminous, will resist the motions of the more dense and still-luminous portions. If we recur to the supposition, which we lately made, that the Sun were expanded into a nebulous sphere, reaching the orbit of Neptune, the diffused matter would offer a far greater resistance to the motions of comets than they now experience. In that case, Encke's comet might be brought to the centre after a few revolutions; and if, while it were thus descending, it were to be drawn out into a string of luminous masses, as Biela's comet has begun to be, these comets, and any others, would form separate luminous spiral tracks in the solar system; and would convert it into a spiral nebula of many branches, like those which are now the most recent objects of astronomical wonder.

29. It seems allowable to regard it as one of those coincidences, in the epochs of related yet seeming unconnected discoveries, which have so often occurred in the history of science; that we should, nearly at the same time, have had brought to our notice, the prevalence of spiral nebulæ, and the circumstances, in Biela's and in Encke's comets, which seem to explain them: the one by showing the origin of luminous broken lines, one part drifting on faster than another, according to its different density, as is usual in incoherent masses;[11] and the other by showing the origin of the spiral form of those lines, arising from the motion being in a resisting medium.

30. But though I have made suppositions by which our Solar System might become a spiral nebula, undoubtedly it is at present something very different; and the leading points of difference are very important for us to consider. And the main point is, that which has already been cursorily noticed: that instead of consisting of matter all nearly of the same density, and a great deal of it luminous, our Solar System consists of kinds of matter immensely different in density, and of large and regular portions which are not luminous. Instead of a diffused nebula with vaporous comets trailing spiral tracks through a medium little rarer than themselves; we have a central sun, and the dark globes of the solid planets rolling round him, in a medium so rare, that in thousands of revolutions not a vestige of retardation can be discovered by the most subtle and persevering researches of astronomers. In the solar system, the luminous matter is collected into the body of the sun; the non-luminous matter, into the planets. And the comets and the resisting medium, which offer a small exception to this account, bear a proportion to the rest which the power of numbers scarce suffices to express.

31. Thus with regard to the density of matter in the solar system; we have supposed, as a mode of expression, that the density of a comet, Encke's comet for instance, is 100,000 times that of the resisting medium. Probably this is greatly understated; and probably also we greatly understate the matter, when we suppose that the tail of a comet is 100,000 times rarer than the matter of the sun.[12] And thus the resisting medium would be, at a very low calculation, 10,000 millions of times more rare than the substance of the sun.

32. And thus we are not, I think, going too far, when we say, that our Solar System, compared with spiral nebulous systems, is a system completed and finished, while they are mere confused, indiscriminate, incoherent masses. In the Nebulæ, we have loose matter of a thin and vaporous constitution, differing as more or less rare, more or less luminous, in a small degree; diffused over enormous spaces, in straggling and irregular forms; moving in devious and brief curves, with no vestige of order or system, or even of separation of different kinds of bodies. In the Solar System, we have the luminous separated from the non-luminous, the hot from the cold, the dense from the rare; and all, luminous and non-luminous, formed into globes, impressed with regular and orderly motions, which continue the same for innumerable revolutions and cycles.[13] The spiral nebulæ, compared with the solar system, cannot be considered as other than a kind of chaos; and not even a chaos, in the sense of a state preceding an orderly and stable system; for there is no indication, in those objects, of any tendency towards such a system. If we were to say that they appear mere shapeless masses, flung off in the work of creating solar systems, we might perhaps disturb those who are resolved to find everywhere worlds like ours; but it seems difficult to suggest any other reason for not saying so.

33. The same may be said of the other very irregular nebulæ, which spread out patches and paths of various degrees of brightness; and shoot out, into surrounding space, faint branches which are of different form and extent, according to the optical power with which they are seen. These irregular forms are incapable of being permanent according to the laws of mechanics. They are not figures of equilibrium; and, therefore, must change by the attraction of the matter upon itself. But if the tenuity of the matter is extreme, and the resistance of the medium in which it floats considerable, this tendency to change and to condensation may be almost nullified; and the bright specks may long keep their straggling forms, as the most fantastically shaped clouds of a summer-sky often do. It is true, it may be said that the reason why we see no change in the form of such nebulæ, is that our observations have not endured long enough; all visible changes in the stars requiring an immense time, according to the gigantic scale of celestial mechanism. But even this hypothesis (it is no more) tends to establish the extreme tenuity of the nebulæ; for more solid systems, like our solar system, require, for the preservation of their form, motions which are perceptible, and indeed conspicuous, in the course of a month; namely, the motions of the planets. All, therefore, concurs to prove the extreme tenuity of the substance of irregular nebulæ.

34. Nebulæ which assume a regular, for instance, a circular or oval shape, with whatever variation of luminous density from the inner to the outer parts, may have a form of equilibrium, if their parts have a proper gyratory motion. Still, we see no reason for supposing that these differ so much from irregular nebulæ, as to be denser bodies, kept in their forms by rapid motions. We are rather led to believe that, though perhaps denser than the spiral nebulæ, they are still of extremely thin and vaporous character. It would seem very unlikely that these vast clouds of luminous vapor should be as dense as the tail of a comet; since a portion of luminous matter so small as such a tail is, must have cooled down from its most luminous condition; and must require to be more dense than nebular matter in order to be visible at all by its own light.

35. Thus we appear to have good reason to believe that nebulæ are vast masses of incoherent or gaseous matter, of immense tenuity, diffused in forms more or less irregular, but all of them destitute of any regular system of solid moving bodies. We seem, therefore, to have made it certain that these celestial objects at least are not inhabited. No speculators have been bold enough to place inhabitants in a comet; except, indeed, some persons who have imagined that such a habitation, carrying its inmates alternately into the close vicinity of the sun's surface, and far beyond the orbit of Uranus, and thus exposing them to the fierce extremes of heat and cold, might be the seat of penal inflictions on those who had deserved punishment by acts done in their life on one of the planets. But even to give coherence to this wild imagination, we must further suppose that the tenants of such prison-houses, though still sensible to human suffering from extreme heat and cold, have bodies of the same vaporous and unsubstantial character as the vehicle in which they are thus carried about the system; for no frame of solid structure could be sustained by the incoherent and varying volume of a comet. And probably, to people the nebulæ with such thin and fiery forms, is a mode of providing them with population, that the most ardent advocates of the plurality of worlds are not prepared to adopt.

36. So far then as the Nebulæ are concerned, the improbability of their being inhabited, appears to mount to the highest point that can be conceived. We may, by the indulgence of fancy, people the summer-clouds, or the beams of the aurora borealis, with living beings, of the same kind of substance as those bright appearances themselves; and in doing so, we are not making any bolder assumption than we are, when we stock the Nebulæ with inhabitants, and call them in that sense, "distant worlds."