The Plurality of Worlds

25. There are other phenomena of the Fixed Stars, and other conjectures of astronomers respecting them, which I need not notice, as they do not appear to have any bearing upon our subject. Such are the "proper motions" of the stars, and the explanation which has been suggested of some of them; that they arise from the stars revolving round other stars which are dark, and therefore invisible. Such again is the attempt to show that the Sun, carrying with it the whole Solar System, is in motion; and the further attempt to show the direction of this motion; and again, the hypothesis that the Sun itself revolves round some distant body in space. These minute inquiries and bold conjectures, as to the movements of the masses of matter which occupy the universe, do not throw any light on the question whether any part besides the earth is inhabited; any more than the investigation of the movements of the ocean, and of their laws, could prove or disprove the existence of marine plants and animals. They do not on that account cease to be important and interesting subjects of speculation; but they do not belong to our subject.

26. In Fontenelle's Dialogues on the Plurality of Worlds, a work which may be considered as having given this subject a place in popular literature, he illustrates his argument by a comparison, which it may be worth while to look at for a moment. The speaker who asserts that the moon, the planets, and the stars, are the seats of habitation, describes the person, who denies this, as resembling a citizen of Paris, who, seeing from the towers of Notre Dame the town of Saint Denis, (it being supposed that no communication between the two places had ever occurred,) denies that it is inhabited, because he cannot see the inhabitants. Of course the conclusion is easy, if we may thus take for granted that what he sees is a town. But we may modify this image, so as to represent our argument more fairly. Let it be supposed that we inhabit an island, from which innumerable other islands are visible; but the art of navigation being quite unknown, we are ignorant whether any of them are inhabited. In some of these islands, are seen masses more or less resembling churches; and some of our neighbors assert that these are churches; that churches must be surrounded by houses; and that houses must have inhabitants. Others hold that the seeming churches are only peculiar forms of rocks. In this state of the debate, everything depends upon the degree of resemblance to churches which the forms exhibit. But suppose that telescopes are invented, and employed with diligence upon the questionable shapes. In a long course of careful and skilful examination, no house is seen, and the rocks do not at all become more like churches, rather the contrary. So far, it would seem, the probability of inhabitants in the islands is lessened. But there are other reasons brought into view. Our island is a long extinct volcano, with a tranquil and fertile soil; but the other islands are apparently somewhat different. Some of them are active volcanoes, the volcanic operations covering, so far as we can discern, the whole island; others undergo changes, such as weather or earthquakes may produce; but in none of them can we discover such changes as show the hand of man. For these islands, it would seem the probability of inhabitants is further lessened. And so long as we have no better materials than these for forming a judgment, it would, surely, be accounted rash, to assert that the islands in general are inhabited; and unreasonable, to blame those who deny or doubt it. Nor would such blame be justified by adducing theological or à priori arguments; as, that the analogy of island with island makes the assumption allowable; or that it is inconsistent with the plan of the Creator of islands to leave them uninhabited. For we know that many islands are, or were long, uninhabited. And if ours were an island occupied by a numerous, well-governed, moral, and religious race, of which the history was known, and of which the relation to the Creator was connected with its history; the assumption of a history, more or less similar to ours, for the inhabitants of the other islands, whose existence was utterly unproved, would, probably, be generally deemed a fitter field for the romance-writer than for the philosopher. It could not, at best, rise above the region of vague conjecture.

27. Fontenelle, in the agreeable book just referred to, says, very truly, that the formula by which his view is urged on adversaries is, Pourquoi non? which he holds to be a powerful figure of logic. It is, however, a figure which has this peculiarity, that it may, in most cases, be used with equal force on either side. When we are asked Why the Moon, Mercury, Saturn, the system of Sirius, should not be inhabited by intelligent beings; we may ask, Why the earth in the ages previous to man might not be so inhabited? The answer would be, that we have proof how it was inhabited. And as to the fact in the other case, I shall shortly attempt to give proof that the Moon is certainly not, and Mercury and Saturn probably not inhabited. With regard to the Fixed Stars, it is more difficult to reason; because we have the means of knowing so little of their structure. But in this case also, we might easily ask on our side, Pourquoi non? Why should not the Solar System be the chief and most complete system in the universe, and the Earth the principal planet in that System? So far as we yet know, the Sun is the largest Sun among the stars; and we shall attempt to show, that the Earth is the largest solid opaque globe in the solar system. Some System must be the largest and most finished of all; why not ours? Some planet must be the largest planet; why not the Earth?

28. It should be recollected that there must be some system which is the most complete of all systems, some planet which is the largest of all planets. And if that largest planet, in the most complete system, be, after being for ages tenanted by irrational creatures, at last, and alone of all, occupied by a rational race, that race must necessarily have the power of asking such questions as these: Why they should be alone rational? Why their planet should be alone thus favored? If the case be ours, we may hope to be then able to answer these questions, when we can explain the most certain fact which they involve; Why the Earth was occupied so long by irrational creatures, before the rational race was placed upon it? The mere power of asking such questions can prove or disprove nothing; for it is a power which must equally subsist, whether the human inhabitants of the earth be or be not the only rational population which the universe contains. If there be a race thus favored by the Creator, they must, at that stage of their knowledge in which man now is, be able to doubt, as man does, of the extent and greatness of the privilege which they enjoy.

29. The argument that the Fixed Stars are like the Sun, and therefore the centres of inhabited systems as the Sun is, is sometimes called an argument from Analogy; and this word Analogy is urged, as giving great force to the reasoning. But it must be recollected, that precisely the point in question is, whether there is an analogy. The stars, it is said, are like the Sun. In what respects? We know of none, except in being self-luminous; and this they have in common with the nebulæ, which, as we have seen, are not centres of inhabited systems. Nor does this quality of being self-luminous at all determine the degree of condensation of a star. Sirius may be less than a hundredth or a thousandth of the density of the Sun. But the Stars, it may be further urged, are like the Sun in turning on their axes. To this we reply, that we know this only of those stars in which, the very phenomenon which proves their revolution, proves also that they are unlike the Sun, in having one side darker than the other. Add to which, their revolution is not connected with the existence of planets, still less of inhabitants of planets, in any intelligible manner. The resemblance, therefore, so far as it bears upon the question, is confined to one single point, in the highest degree ambiguous and inconclusive; and any argument drawn from this one point of resemblance, has little claim to be termed an argument from analogy.41

30. On a subject on which we know so little, it is difficult to present any view which deserves to be regarded as an analogy. We see, among the stars, nebulæ more or less condensed, which are possibly, in some cases, stages of a connected progress towards a definite star; and it may be, to a star with planets in permanent orbits. We see, in our planet, evidence of successive stages of a connected series of brute animals, preceded perhaps by various stages of lifeless chaos. If the histories of the Sun, and of all the stars, are governed by a common analogy, the nebulous condensation, and the stages of animal life, may be parts of the same continued series of events; and different stars may be at different points of that series. But even on this supposition, but a few of the stars may be the seats of conscious life, and none, of intelligence. For among the stars which have condensed to a permanent form, how many have failed in throwing off a permanent planet! How many may be in some stage of lifeless chaos! We must needs suppose a vast number of stages between a nebular chaos and the lowest forms of conscious life. Perhaps as many as there are fixed stars; and far more than there are of stars which become fertile of life: so that no two systems may be at the same stage of the planetary progress. And if this be so,—our system being so complicated, that we must suppose it peculiarly developed, having the largest Sun that we know of, and our Earth being (as we shall hereafter attempt to prove) the largest solid planet that we know of,—this Earth may be the sole seat of the highest stage of planetary development.

31. The assumption that there is anything of the nature of a regular law or order of progress from nebular matter to conscious life,—a law which extends to all the stars, or to many of them,—is in the highest degree precarious and unsupported; but since it is sometimes employed in such speculations as we are pursuing, we may make a remark or two connected with it. If we suppose, on the planets of other systems, a progress in some degree analogous to that which geology shows to have occurred on the Earth, there may be, in those planets, creatures in some way analogous to our vegetables and animals; but analogy also requires that they should differ far more from the terrestrial vegetables and animals of any epoch, than those of one epoch do from those of another; since they belong to a different stellar system, and probably exist under very different conditions from any that ever prevailed on the Earth. We are forbidden, therefore, by analogy, to suppose that on any other planet there was such an anatomical progression towards the form of man, as we can discern (according to some eminent physiologists) among the tribes which have occupied the Earth. Are we to conceive that the creatures on the planets of other systems are, like the most perfect terrestrial animals, symmetrical as to right and left, vertebrate, with fore limbs and hind limbs, heads, organs of sense in their heads, and the like? Every one can see how rash and fanciful it would be to make such suppositions. Those who have, in the play of their invention, imagined inhabitants of other planets, have tried to avoid this servile imitation of terrestrial forms. Here is Sir Humphry Davy's account of the inhabitants of Saturn. "I saw moving on the surface below me, immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to that of the morse or sea-horse, but I saw with great surprise that they moved from place to place by six extremely thin membranes, which they used as wings. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant, than to anything else I can imagine, occupying what I supposed to be the upper parts of the body."42 The attendant Genius informs the narrator, that though these creatures look like zoophytes, they have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of the Earth. If we were to reason upon a work of fancy like this, we might say, that it was just as easy to ascribe superior sensibility and intelligence to zoophyte-formed creatures upon the Earth, as in Saturn. Even fancy cannot aid us in giving consistent form to the inhabitants of other planets.

32. But even if we could assent to the opinion, as probable, that there may occur, on some other planet, progressions of organized forms analogous in some way to that series of animal forms which has appeared upon the earth, we should still have no ground to assume that this series must terminate in a rational and intelligent creature like man. For the introduction of reason and intelligence upon the Earth is no part nor consequence of the series of animal forms. It is a fact of an entirely new kind. The transition from brute to man does not come within the analogy of the transition from brute to brute. The thread of analogy, even if it could lead us so far, would break here. We may conceive analogues to other animals, but we could have no analogue to man, except man. Man is not merely a higher kind of animal; he is a creature of a superior order, participating in the attributes of a higher nature; as we have already said, and as we hope hereafter further to show. Even, therefore, if we were to assume the general analogy of the Stars and of the Sun, and were to join to that the information which geology gives us of the history of our own planet; though we might, on this precarious path, be led to think of other planets as peopled with unimagined monsters; we should still find a chasm in our reasoning, if we tried, in this way, to find intelligent and rational creatures in planets which may revolve round Sirius or Arcturus.

33. The reasonable view of the matter appears to be this. The assumption that the Fixed Stars are of exactly the same nature as the Sun, was, at the first, when their vast distance and probable great size were newly ascertained, a bold guess; to be confirmed or refuted by subsequent observations and discoveries. Any appearances, tending in any degree to confirm this guess, would have deserved the most considerate attention. But there has not been a vestige of any such confirmatory fact. No planet, nor anything which can fairly be regarded as indicating the existence of a planet, revolving about a star, has anywhere been discerned. The discovery of nebulæ, of binary systems, of clusters of stars, of periodical stars, of varying and accelerated periods of such stars, all seem to point the other way. And if all these facts be held to be but small in amount, as to the information which they convey, about the larger, and perhaps nearer stars; still they leave the original assumption a mere guess, unsupported by all that three centuries of most diligent, and in other respects successful research, has been able to bring to light. That Copernicus, that Galileo, that Kepler, should believe the stars to be Suns, in every sense of the term, was a natural result of the expansion of thought which their great discoveries produced, in them and in their contemporaries. Nor are we yet called upon to withdraw from them our sympathy; or entitled to contradict their conjecture. But all the knowledge that the succeeding times have given us; the extreme tenuity of much of the luminous matter in the skies; the existence of gyratory motion among the stars, quite different from planetary systems; the absence of any observed motions at all resembling such systems; the appearance of changes in stars, quite inconsistent with such permanent systems; the disclosure of the history of our own planet, as one in which changes have constantly been going on; the certainty that by far the greater part of the duration of its existence, it has been tenanted by creatures entirely different from those which give an interest, and thence, a persuasiveness, to the belief of inhabitants in worlds appended to each star; the impossibility, which appears, on the gravest consideration, of transferring to other worlds such interests as belong to our own race in this world; all these considerations should, it would seem, have prevented that old and arbitrary conjecture from growing up, among a generation professing philosophical caution, and scientific discipline, into a settled belief.

34. Some of the moral and theological views which tend to encourage and uphold this belief, may be taken under our more special consideration hereafter: but here, where we are reasoning principally upon astronomical grounds, we may conclude what we have to remark about the Fixed Stars, as the centres of inhabited systems of worlds, by saying; that it will be time enough to speculate about the inhabitants of the planets which belong to such systems, when we have ascertained that there are such planets, or one such planet. When that is done, we can then apply to them any reasons which may exist, for believing that all, or many planets, are the seats of habitation of living things. What reasons of this kind can be adduced, and what is their force with regard to our own solar system, we must now proceed to discuss.43

CHAPTER IX

THE PLANETS

1. When it was discovered, by Copernicus and Galileo, that Mercury, Venus, Mars, Jupiter, Saturn, which had hitherto been regarded only as "wandering fires, that move in mystic dance," were really, in many circumstances, bodies resembling the Earth;—that they and the Earth alike, were opaque globes, revolving about the Sun in orbits nearly circular, revolving also about their own axes, and some of them accompanied by their Satellites, as the Earth is by the Moon;—it was inevitable that the conjecture should arise, that they too had inhabitants, as the Earth has. Each of these bodies were seemingly coherent and solid; furnished with an arrangement for producing day and night, summer and winter; and might therefore, it was naturally conceived, have inhabitants moving upon its solid surface, and reckoning their lives and their employment by days, and months, and years. This was an unavoidable guess. It was far less bold and sweeping than the guess that there are inhabitants in the region of the Fixed Stars, but still, like that, it was, for the time at least, only a guess; and like that, it must depend upon future explorations of these bodies and their conditions, whether the guess was confirmed or discredited. The conjecture could not, by any moderately cautious man, be regarded as so overwhelmingly probable, that it had no need of further proof. Its final acceptance or rejection must depend on the subsequent progress of astronomy, and of science in general.

2. We have to consider then how far subsequent discoveries have given additional value to this conjecture. And, as, in the first place, important among such discoveries, we must note the addition of several new planets to our system. It was found, by the elder Herschel, (in 1781,) that, far beyond Saturn, there was another planet, which, for a time, was called by the name of its sagacious discoverer; but more recently, in order to conform the nomenclature of the planets to the mythology with which they had been so long connected, has been termed Uranus. This was a vast extension of the limits of the solar system. The Earth is, as we have already said, nearly a hundred millions of miles from the Sun. Jupiter is at more than five times, and Saturn nearly at ten times this distance: but Uranus, it was found, describes an orbit of which the radius is about nineteen times as great as that of the Earth. But this did not terminate the extension of the solar system which the progress of astronomy revealed. In 1846, a new planet, still more remote, was discovered: its existence having been divined, before it was seen, by two mathematicians, Mr. Adams, of Cambridge, and M. Leverrier, of Paris, from the effects of its force upon Uranus. This new planet was termed Neptune: its distance from the Sun is about thirty times the Earth's distance. Besides these discoveries of large planets, a great number of small planets were detected in the region of the solar system which lies between the orbits of Mars and Jupiter. This series of discoveries began on the first day of 1801, when Ceres was detected by Piazzi at Palermo; and has gone on up to the present time, when twenty-three of these small bodies have been brought to light; and probably the group is not yet exhausted.

3. Now if we have to discuss the probability that all these bodies are inhabited, we may begin with the outermost of them at present known, namely Neptune. How far is it likely that this globe is occupied by living creatures which enjoy, like the creatures on the Earth, the light and heat of the Sun, about which the planet revolves? It is plain, in the first place, that this light and heat must be very feeble. Since Neptune is thirty times as far from the sun as the earth is, the diameter of the sun as seen from Neptune will only be one-thirtieth as large as it is, seen from the earth. It will, in fact, be reduced to a mere star. It will be about the diameter under which Jupiter appears when he is nearest to us. Of course its brightness will be much greater than that of Jupiter; nearly as much indeed, as the sun is brighter than the moon, both being nearly of the same size: but still, with our full-moonlight reduced to the amount of illumination which we receive from a full Jupiter, and our sun-light reduced in nearly the same proportion, we should have but a dark, and also a cold world. In fact, the light and the heat which reach Neptune, so far as they depend on the distance of the sun, will each be about nine hundred times smaller than they are on the earth. Now are we to conceive animals, with their vital powers unfolded, and their vital enjoyments cherished, by this amount of light and heat? Of course, we cannot say, with certainty, that any feebleness of light and heat are inconsistent with the existence of animal life: and if we had good reason to believe that Neptune is inhabited by animals, we might try to conceive in what manner their vital scheme is accommodated to this scanty supply of heat and light. If it were certain that they were there, we might inquire how they could live there, and what manner of creatures they could be. If there were any general grounds for assuming inhabitants, we might consider what modifications of life their particular conditions would require.

4. But is there any such general ground!? Such a ground we should have, if we could venture to assume that all the bodies of the Solar System are inhabited;—if we could proceed upon such a principle, we might reject or postpone the difficulties of particular cases.

5. But is such an assumption true? Is such a principle well founded? The best chance which we have of learning whether it is so, is to endeavor to ascertain the fact, in the body which is nearest to us; and thus, the best placed for our closer scrutiny. This is, of course, the Moon; and with regard to the Moon, we have, again, this advantage in beginning the inquiry with her:—that she, at least, is in circumstances, as to light and heat, so far as the Sun's distance affects them, which we know to be quite consistent with animal and vegetable life. For her distance from the Sun is not appreciably different from that of the Earth; her revolutions round the earth do not make nearly so great a difference, in her distance from the sun, as does the earth's different distances from the sun in summer and in winter: the fact also being, that the earth is considerably nearer to the sun in the winter of this our northern hemisphere, than in the summer. The moon's distance from the sun then, adapts her for habitation: is she inhabited?

6. The answer to this question, so far as we can answer it, may involve something more than those mere astronomical conditions, her distance from the sun, and the nature of her motions. But still, if we are compelled to answer it in the negative;—if it appear, by strong evidence, that the Moon is not inhabited; then is there an end of the general principle, that, all the bodies of the solar system are inhabited, and that we must begin our speculations about each, with this assumption. If the Moon be not inhabited, then, it would seem, the belief that each special body in the system is inhabited, must depend upon reasons specially belonging to that body; and cannot be taken for granted without such reasons. Of the two bodies of the solar system which alone we can examine closely, so as to know anything about them, the Earth and the Moon, if the one be inhabited, and the other blank of inhabitants, we have no right to assume at once, that any other body in the solar system belongs to the former of these classes rather than to the latter. If, even under terrestrial conditions of light and heat, we have a total absence of the phenomenon of life, known to us only as a terrestrial phenomenon; we are surely not entitled to assume that when these conditions fail, we have still the phenomenon, life. We are not entitled to assume it; however it may be capable of being afterwards proved, in any special case, by special reasons; a question afterwards to be discussed.