The Plurality of Worlds

by William Whewell

20. Perhaps it may occur to some one to ask, if this planet, which presents so glorious an aspect to our eyes, be thus the abode only of such imperfect and embryotic lumps of vitality as I have described; to what purpose was all that gorgeous array of satellites appended to him, which would present, to intelligent spectators on his surface, a spectacle far more splendid than any that our skies offer to us: four moons, some as great, and others hardly less, than our moon, performing their regular revolutions in the vault of heaven. To which it will suffice, at present, to reply, that the use of those moons, under such a supposition, would be precisely the same, as the use of our moon, during the myriads of years which elapsed while the earth was tenanted by corals and madrepores, shell-fish and belemnites, the cartilaginous fishes of the Old Red Sandstone, or the Saurian monsters of the Lias; and in short, through all the countless ages which elapsed, before the last few thousand years: before man was placed upon the earth "to eye the blue vault and bless the useful light:" to reckon by it his months and years: to discover by means of it, the structure of the universe, and perhaps, the special care of his Creator for him alone of all his creatures. The moons of Jupiter, may in this way be of use, as our own moon is. Indeed we know that they have been turned to most important purposes, in astronomy and navigation. And knowing this, we may be content not to know how, either the satellites of Jupiter, or the satellite of the Earth, tend to the advantage of the brute inhabitants of the waters.

21. There is another point, connected with this doctrine of the watery nature of Jupiter, which I may notice, though we have little means of knowledge on the subject. Jupiter being thus covered with water, is the water ever converted into ice? The planet is more than 5 times as far from the sun as the earth is: the heat which he receives is, on that account, 25 times less than ours. The veil of clouds which covers a large part of his surface, must diminish the heat still further. What effect the absence of land produces, on the freezing of the ocean, it is not easy to say. We cannot, therefore, pronounce with any confidence whether his waters are ever frozen or not. In the next considerable planet, Mars, astronomers conceive that they do trace the effects of frost; but in Mars we have also appearances of land. In Jupiter, we are left to mere conjecture; whether continents and floating islands of ice still further chill the fluids of the slimy tribes whom we have been led to regard as the only possible inhabitants; or whether the watery globe is converted into a globe of ice; retaining on its surface, of course, as much fluid as is requisite, under the evaporating power of the sun, to supply the currents of vapor which form the belts. In this case, perhaps, we may think it most likely that there are no inhabitants of these shallow pools in a planet of ice: at any rate, it is not worth while to provide any new speculations for such a hypothesis.

22. We may turn our consideration from Jupiter to Saturn; for in many respects the two planets are very similar. But in almost every point, which is of force against the hypothesis of inhabitants, the case is much stronger in Saturn than it is in Jupiter. Light and heat, at his distance, are only one ninetieth of those at the Earth. None but a very low degree of vitality can be sustained under such sluggish influences. The density of his mass is hardly greater than that of cork; much less than that of water: so that, it does not appear what supposition is left for us, except that a large portion of the globe, which we see as his, is vapor. That the outer part of the globe is vapor, is proved, in Saturn as in Jupiter, by the existence of several cloudy streaks or belts running round him parallel to his equator. Yet his mass, taken altogether, is considerable, on account of his great size; and gravity would be greater, at his outer surface, than it is at the earth's. For such reasons, then, as were urged in the case of Jupiter, we must either suppose that he has no inhabitants; or that they are aqueous, gelatinous creatures; too sluggish, almost to be deemed alive, floating on their ice-cold waters, shrouded forever by their humid skies.

23. Whether they have eyes or no, we cannot tell; but probably if they had, they would never see the Sun; and therefore we need not commiserate their lot in not seeing the host of Saturnian satellites; and the Ring, which to an intelligent Saturnian spectator, would be so splendid a celestial object. The Ring is a glorious object for man's view, and his contemplation; and therefore is not altogether without its use. Still less need we (as some appear to do) regard as a serious misfortune to the inhabitants of certain regions of the planet, a solar eclipse of fifteen years' duration, to which they are liable by the interposition of the Ring between them and the Sun.[10]

24. The cases of Uranus and Neptune are similar to that of Saturn, but of course stronger, in proportion to their smaller light and heat. For Uranus, this is only 1-360th, for Neptune, as we have already said, 1-900th of the light and heat at the earth. Moreover, these two new planets agree with Jupiter and with Saturn, in being of very large size and of very small density; and also we may remark, one of them, probably both, in revolving with great rapidity, and in nearly the same period, namely, about 10 hours: at least, this has been the opinion of astronomers with regard to Uranus. The arguments against the hypothesis of these two planets being inhabited, are of course of the same kind as in the case of Jupiter and Saturn, but much increased in strength; and the supposition of the probably watery nature and low vitality of their inhabitants must be commended to the consideration of those who contend for inhabitants in those remote regions of the solar system.

25. We may now return towards the Sun, and direct our attention to the planet Mars. Here we have some approximation to the condition of the Earth, in circumstances, as in position. It is true, his light and heat, so far as distance from the Sun affects them, are less than half those at the Earth. His density appears to be nearly equal to that of the Earth, but his mass is so much smaller, that gravity at his surface is only one-half of what it is here. Then, as to his physical condition, so far as we can determine it, astronomers discern in his face[11] the outlines of continents and seas. The ruddy color by which he is distinguished, the red and fiery aspect which he presents, arise, they think, from the color of the land, while the seas appear greenish. Clouds often seem to intercept the astronomer's view of the globe, which with its continents and oceans thus revolves under his eye; and that there is an atmosphere on which such clouds may float, appears to be further proved, by brilliant white spots at the poles of the planet, which are conjectured to be snow; for they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of their polar winter; the snow-line then extending to about six degrees (reckoned upon the meridian of the planet) from the pole. Moreover, Mars agrees with the earth, in the period of his rotation; which is about 24 hours; and in having his axis inclined to his orbit, so as to produce a cycle of long and short days and nights, a return of summer and winter, in every revolution of the planet.

26. We have here a number of circumstances which speak far more persuasively for a similarity of condition, in this planet and the Earth, than in any of the cases previously discussed. It is true, Mars is much smaller than the earth, and has not been judged worthy of the attendance of a satellite, although further from the Sun; but still, he may have been judged worthy of inhabitants by his Creator. Perhaps we are not quite certain about the existence of an atmosphere; and without such an appendage, we can hardly accord him tenants. But if he have inhabitants, let us consider of what kind they must be conceived to be, according to any judgment which we can form. The force of his gravity is so small, that we may allow his animals to be large, without fearing that they will break down by their own weight. In a planet so dense, they may very likely have solid skeletons. The ice about his poles will cumber the seas, cold even for the want of solar heat, as it does in our arctic and antarctic oceans; and we may easily imagine that these seas are tenanted, like those, by huge creatures of the nature of whales and seals, and by other creatures which the existence of these requires and implies. Or rather, since, as we have said, we must suppose the population of other planets to be more different from our existing population, than the population of other ages of our own planet, we may suppose the population of the seas and of the land of Mars, (if there be any, and if we are not carrying it too high in the scale of vital activity,) to differ from any terrestrial animals, in something of the same way in which the great land and sea saurians, or the iguanodon and dinotherium, differed from the animals which now live on the earth.

27. That we need not discuss the question, whether there are intelligent beings living on the surface of Mars, perhaps the reader will allow, till we have some better evidence that there are living things there at all; if he calls to mind the immense proportion which, on the earth, far better fitted for the habitation of the only intelligent creature which we know or can conceive, the duration of unintelligent life has borne to that of intelligent. Here, on this Earth, a few thousand years ago, began the life of a creature who can speculate about the past and the future, the near and the absent, the Universe and its Maker, duty and immortality. This began a few thousand years ago, after ages and myriads of ages, after immense varieties of lives and generations, of corals and mollusks, saurians, iguanodons, and dinotheriums. No doubt the Creator might place an intelligent creature upon a planet, without all this preparation, all this preliminary life. He has not chosen to do so on the earth, as we know; and that is by much the best evidence attainable by us, of what His purposes are. It is also possible that He should, on another planet, have established creatures of the nature of corals and mollusks, saurians and iguanodons, without having yet arrived at the period of intelligent creatures: especially if that other planet have longer years, a colder climate, a smaller mass, and perhaps no atmosphere. It is also possible that He should have put that smaller planet near the Earth, resembling it in some respects, as the Moon does, but without any inhabitants, as she has none; and that Mars may be such a planet. The probability against such a belief can hardly be considered as strong, if the arguments already offered be regarded as effective against the opinion of inhabitants in the other planets, and in the Moon.

28. The numerous tribe of small bodies, which revolve between Jupiter and Mars, do not admit of much of the kind of reasoning, which we have applied to the larger planets. They have, with perhaps one exception (Vesta) no disk of visible magnitude; they are mere dots, and we do not even know that their form is spherical. The near coincidence of their orbits has suggested, to astronomers, the conjecture that they have resulted from the explosion of a larger body, and from its fracture into fragments. Perhaps the general phenomena of the universe suggest rather the notion of a collapse of portions of sidereal matter, than of a sudden disruption and dispersion of any portion of it; and these small bodies may be the results of some imperfectly effected concentration of the elements of our system; which, if it had gone on more completely and regularly, might have produced another planet, like Mars or Venus. Perhaps they are only the larger masses, among a great number of smaller ones, resulting from such a process: and it is very conceivable, that the meteoric stones which, from time to time, have fallen upon the earth's surface, are other results of the like process:—bits of planets which have failed in the making, and lost their way, till arrested by the resistance of the earth's atmosphere. A remarkable circumstance in these bodies is, that though thus coming apparently from some remote part of the system, they contain no elements but such as had already been found to exist in the mass of the earth; although some substances, as nickel and chrome, which are somewhat rare in the earth's materials, are common parts of the composition of meteoric stones. Also they are of crystalline structure, and exhibit some peculiarities in their crystallization. Such as these strange visitors are, they seem to show that the other parts of the solar system contain the same elementary substances, and are subject to the same laws of chemical synthesis and crystalline force, which obtain in the terrestrial region. The smallness of these specimens is a necessary condition of their reaching us; for if they had been more massive, they would have followed out the path of their orbits round the sun, however eccentric these might be. The great eccentricity of the smaller planets, their great deviation from the zodiacal path, which is the highway of the large planets, their great number, probably by no means yet exhausted by the discoveries of astronomers; all fall in with the supposition that there are, in the solar system, a vast multitude of such abnormal planetoidal lumps. As I have said, we do not even know that they are approximately spherical; and if they are of the nature of meteoric stones, they are mere crude and irregularly crystallized masses of metal and earth. It will therefore, probably, be deemed unnecessary to give other reasons why these planetoids are not inhabited. But if it be granted that they are not, we have here, in addition to the moon, a large array of examples, to prove how baseless is the assumption, that all the bodies of the solar system are the seats of life.

29. We have thus performed our journey from the extremest verge of the Universe, so far as we have any knowledge of it, to the orbit of our own planet; and have found, till we came into our own most immediate vicinity, strong reasons for rejecting the assumption of inhabited worlds like our own; and indeed, of the habitation of worlds in any sense. And even if Mars, in his present condition, may be some image of the Earth, in some of its remote geological periods, it is at least equally possible that he may be an image of the Earth, in the still remoter geological period before life began. Of peculiar fitnesses which make the earth suited to the sustentation of life, as we know that it is, we shall speak hereafter; and at present pass on to the other planets, Venus and Mercury. But of these, there is, in our point of view, very little to say. Venus, which, when nearest to us, fills a larger angle than any other celestial body, except the Sun and the Moon, might be expected to be the one of which we know most. Yet she is really one of the most difficult to scrutinize with our telescopes. Astronomers cannot discover in her, as in Mars, any traces of continents and seas, mountains and valleys; at least with any certainty.[12] Her illuminated part shines with an intense lustre which dazzles the sight;[13] yet she is of herself perfectly dark; and it was the discovery, that she presented the phases of the Moon, made by the telescope of Galileo, which gave the first impulse to planetary research. She is almost as large as the earth; almost as heavy. The light and heat which she receives from the Sun must be about double those which come to the earth. We discern no traces of a gaseous or watery atmosphere surrounding her. Perhaps if we could see her better, we might find that she had a surface like the moon; or perhaps, in the nearer neighborhood of the sun, she may have cooled more slowly and quietly, like a glass which is annealed in the fire; and hence, may have a smooth surface, instead of the furrowed and pimpled visage which the Moon presents to us. With this ignorance of her conditions, it is hard to say what kind of animals we could place in her, if we were disposed to people her surface; except perhaps the microscopic creatures, with siliceous coverings, which, as modern explorers assert, are almost indestructible by heat. To believe that she has a surface like the earth, and tribes of animals, like terrestrial animals, and like man, is an exercise of imagination, which not only is quite gratuitous, but contrary to all the information which the telescope gives us; and with this remark, we may dismiss the hypothesis.