The Plurality of Worlds

by William Whewell

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.

7. Is, then, the Moon inhabited? From the moon's proximity to us, (she is distant only thirty diameters of the earth, less than ten times the earth's circumference; a railroad carriage, at its ordinary rate of travelling, would reach her in a month,) she can be examined by the astronomer with peculiar advantages. The present powers of the telescope enable him to examine her mountains as distinctly as he could the Alps at a few hundred miles distance, with the naked eye; with the additional advantage that her mountains are much more brilliantly illuminated by the Sun, and much more favorably placed for examination, than the Alps are. He can map and model the inequalities of her surface, as faithfully and exactly as he can those of the surface of Switzerland. He can trace the streams that seem to have flowed from eruptive orifices over her plains, as he can the streams of lava from the craters of Etna or Hecla.

8. Now, this minute examination of the Moon's surface being possible, and having been made, by many careful and skilful astronomers, what is the conviction which has been conveyed to their minds, with regard to the fact of her being the seat of vegetable or animal life? Without exception, it would seem, they have all been led to the belief, that the Moon is not inhabited; that she is, so far as life and organization are concerned, waste and barren, like the streams of lava or of volcanic ashes on the earth, before any vestige of vegetation has been impressed upon them: or like the sands of Africa, where no blade of grass finds root. It is held, by such observers, that they can discern and examine portions of the moon's surface as small as a square mile;[1] yet, in their examination, they have never perceived any alteration, such as the cycle of vegetable changes through the revolutions of seasons would produce. Sir William Herschel did not doubt that if a change had taken place on the visible part of the Moon, as great as the growth or the destruction of a great city, as great, for instance, as the destruction of London by the great fire of 1666, it would have been perceptible to his powers of observation. Yet nothing of the kind has ever been observed. If there were lunar astronomers, as well provided as terrestrial ones are, with artificial helps of vision, they would undoubtedly be able to perceive the differences which the progress of generations brings about on the surface of our globe; the clearing of the forests of Germany or North America; the embankment of Holland; the change of the modes of culture which alter the color of the ground in Europe; the establishment of great nests of manufactures which shroud portions of the land in smoke, as those which have their centres at Birmingham or at Manchester. However obscurely they might discern the nature of those changes, they would still see that change was going on. And so should we, if the like changes were going on upon the face of the Moon. Yet no such changes have ever been noticed. Nor even have such changes been remarked, as might occur in a mere brute mass without life;—the formation of new streams of lava, new craters, new crevices, new elevations. The Moon exhibits strong evidences, which strike all telescopic observers, of an action resembling, in many respects, volcanic action, by which its present surface has been formed.[2] But, if it have been produced by such internal fires, the fires seem to be extinguished; the volcanoes to be burned out. It is a mere cinder; a collection of sheets of rigid slag, and inactive craters. And if the Moon and the Earth were both, at first in a condition in which igneous eruptions from their interior produced the ridges and cones which roughen their surfaces; the Earth has had this state succeeded by a series of states of life in innumerable forms, till at last it has become the dwelling-place of man; while the Moon, smaller in dimensions, has at an earlier period completely cooled down, as to its exterior at least, without ever being judged fit or worthy by its Creator of being the seat of life; and remains, hung in the sky, as an object on which man may gaze, and perhaps, from which he may learn something of the constitution of the universe; and among other lessons this; that he must not take for granted, that all the other globes of the solar system are tenanted, like that on which he has his appointed place.

9. It is true, that in coming to this conclusion, the astronomers of whom I speak, have been governed by other reasons, besides those which I have mentioned, the absence of any changes, either rapid or slow, discoverable in the Moon's face. They have seen reason to believe that water and air, elements so essential to terrestrial life, do not exist in the Moon. The dark spaces on her disk, which were called seas by those who first depicted them, have an appearance inconsistent with their being oceans of water. They are not level and smooth, as water would be; nor uniform in their color, but marked with permanent streaks and shades, implying a rigid form. And the absence of an atmosphere of transparent vapor and air, surrounding the moon, as our atmosphere surrounds the earth, is still more clearly proved, by the absence of all the optical effects of such an atmosphere, when stars pass behind the moon's disk, and by the phenomena which are seen in solar eclipses, when her solid mass is masked by the Sun.[3] This absence of moisture and air in the Moon, of course, entirely confirms our previous conclusion, of the absence of vegetable and animal life; and leaves us, as we have said, to examine the question for the other bodies, on their special grounds, without any previous presumption that such life exists. Undoubtedly the aspect of the case will be different in one feature, when we see reason to believe that other bodies have an atmosphere; and if there be in any planet sufficient light and heat, and clouds and winds, and a due adjustment of the power of gravity, and the strength of the materials of which organized frames consist, there may be, so far as we can judge, life of some kind or other. But yet, even in those cases, we should be led to judge also, by analogy, that the life which they sustain is more different from the terrestrial life of the present period of the earth, than that is from the terrestrial life of any former geological period, in proportion as the conditions of light and heat, and attraction and density, are more different on any other planet, than they can have been on the earth, at any period of its history.

10. Let us then consider the state of these elements of being in the other planets. I have mentioned, among them, the force of gravity, and the density of materials; because these are important elements in the question. It may seem strange, that we are able, not only to measure the planets, but to weigh them; yet so it is. The wonderful discovery of universal gravitation, so firmly established, as the law which embraces every particle of matter in the solar system, enables us to do this, with the most perfect confidence. The revolutions of the satellites round their primary planets, give us a measure of the force by which the planets retain them in their orbits; and in this way, a measure of the quantity of matter of which each planet consists. And other effects of the same universal law, enable us to measure, though less easily and less exactly, the masses, even of those planets which have no satellites. And thus we can, as it were, put the Earth, and Jupiter or Saturn, in the balance against each other; and tell the proportionate number of pounds which they would weigh, if so poised. And again, by another kind of experiment, we can, as we have said, weigh the earth against a known mountain; or even against a small sphere of lead duly adjusted for the purpose. And this has been done; and the results are extremely curious; and very important in our speculations relative to the constitution of the universe.