Are the Planets Inhabited?

by E. Walter Maunder

If this thought be sound, it places before us an entirely new and most serious consideration. The world predestined for habitation must not only have its size within certain narrow limits, its distance from its central sun in a certain narrow zone, its rotation period, the inclination of its axis, the eccentricity of its orbit, all suitable alike, but even if in these and in all other necessaries it is perfectly adapted for habitation, yet it will be only during a relatively small fraction of its entire duration that Intelligent Life, clothed in material form, will find a place upon it.

Let us sum shortly what we know and what we conclude. We know that this, our Earth, is a habitable globe, for we ourselves are living upon it. We know what constitutes the physical basis of our life, and under what conditions on this Earth it flourishes, and under what conditions it is destroyed. If we turn our eyes from this, our Earth, and look out upon the starry skies, we see the other planets of our system, and the suns which are the centres of other systems. From the consideration of the planets in our own system, we have seen how stringent and how many are the conditions imposed for Life to be possible. Round our Sun there is but a narrow zone in which a habitable world may circle; in this zone there is room for but few worlds, and we actually know of three alone, the Earth, the Moon, and Venus. We know that the Earth can be and is inhabited; that the Moon is not and cannot be inhabited; and that Venus, though of habitable size, may yet be subject to the fatal disqualification of always turning the same face to the Sun. Of other planetary systems than our own, we actually know of none, but we assume that there are such, and as numerous as there are suns in the starry depths. But of these planetary systems we can rule out, as containing no habitable member, all such as circle round double or multiple suns or, indeed, round any single star that, from whatever cause, is largely variable and, therefore, much less stable than our own. Mira Ceti, which in 5 months increases its brightness 1000 times, may stand as an example. Probably these disqualifications rule out of court the great proportion of the stellar systems. Of the few, comparatively speaking, single and stable suns that remain in the heavenly abyss, we must conclude, from what we know of our solar system, that they, too, have but a narrow zone, outside of which no world would be fit to dwell in; whilst in the zone the few worlds which might exist must violate no one of many strict conditions. If we assume that there are a hundred million stars within the ken of our telescopes, we may well believe that not more than one in a hundred of these would fulfil the condition of being a single and stable sun, such as ours. Of the planets revolving round these million suns—stable and efficient suns—can we expect that in more cases than one in a hundred there will be a planet in the habitable zone fulfilling all the other conditions of habitability, of size, mass, inclination of axis, circular orbit, and rotation? Of these ten thousand earths which may be made fit for the habitation of Man, can we assume that even one in a hundred is now at that epoch in its history when it is no longer “without form and void,” when a division has been made between the waters under the firmament and those that are above the firmament; when the waters under the heaven have been gathered into one place, and the dry land has appeared, and when the earth and the waters have brought forth life abundantly? Out of a hundred million of planetary systems throughout the depths of space, can we suppose that there are even one hundred worlds that are actually inhabited at the present moment? These numbers and proportions certainly are not, and cannot be, based on knowledge; they are given as illustrations only; but, vague as they are, they suggest that our Earth may be neither one of many inhabited earths, nor yet unique, but one of a few—indeed of a very few.

And then the objection is raised: “If our own Earth is but one of, perhaps, two inhabited worlds in the solar system; and of perhaps one or two hundred inhabited worlds throughout the furthest space that we can scan; why is all this waste?” Of all the countless millions of stellar systems without living organisms as inhabitants, we cannot tell the purpose for the simple reason that we do not know it; but of “waste” in the solar system, there is no question. Relatively speaking, this is quite insignificant, for we cannot consider that as “waste material” which is useful and, indeed, essential to existence. For, consider first the material in the Earth itself. Its total volume is 260,613,000,000 cubic miles, but man only lives upon its surface of less than 200 million square miles in extent, and he can not probe down as far as ten miles below it, through the depths of ocean or by his deepest mine. Thus we are left with over 258 thousand million of cubic miles that man, or plant, or beast can never make direct use of. But without this 258 thousand million cubic miles that he can never sow nor reap, the overlying platform on which he dwells would be useless for retaining the air or the water by which he lives. No less essential is the Sun; its vast bulk of

2,000,000,000,000,000,000,000,000,000 tons

can, in no single unit, be counted “waste,” for it is from this that the heat and light necessary for life on the Earth is derived. But the tonnage of all the planets combined is but 0·13 per cent of the Sun alone; and a wastage, if such it is, like this is insignificant from a material point of view.

There is a type of politician at the present day who is convinced that the highest purpose to which land can be put is to build upon it; that being, in general, the use giving the highest money return per square foot, though the return does not always fall to the builder. It has taken not a little agitation and popular pressure to enforce the truth that cultivated land is also of use. But there are few who realize that land that is neither built upon nor cultivated is also essential. Our barren moors and bleak hillsides, “wastelands” as we call them, are absolutely necessary as collectors of the water by which we live. From them our springs take their source; and they supply our cities with the first necessity of life.

We find, then, in this universe so far as we can know it, that Space is lavishly provided, Matter is lavishly scattered, Time is unsparingly drawn upon, but Life in any form, and especially in its highest form, is, relatively speaking, very sparsely given. That very circumstance surely points to the overwhelming importance of conscious, intelligent Life, and the insignificance of lifeless matter in comparison with it. We have to exhaust arithmetic in computing the size, the mass, the output of heat and light of our Sun, yet it is but the hearth-fire and lamp of terrestrial life; and its amazing agglomeration of matter and energy is ungrudgingly devoted to this humble purpose. Whatever view we hold as to the scheme of the universe; whether with the unthinking we fail to recognize Thought and Purpose behind its marvellous manifestations, or, with the thoughtful, realize that only Infinite Thought could provide so wonderfully for the bringing forth of thought in living material organisms, the conclusion still remains: living intelligences are, by the direct testimony of the universe itself, its noblest and most precious product.

The plea is often made that as we find life adapting itself to a great variety of conditions on this Earth, we must not set limits to its power of adaption to the conditions of other worlds. But this plea is an unthinking one. The range of conditions through which we find life on this Earth is as nothing to the range given by the varied sizes and positions of the different planets; and even on our Earth, life in the unfavoured regions—the tops of mountains, the polar snows, the waterless deserts, the ocean depths—is only possible because there are more favoured regions close at hand, and there are, as it were, “crumbs that fall from the rich man’s table.” A well-known littérateur in setting forth “a hundred ways of making money” gave great prominence to the method of living as caretaker in an empty house. But residing in an empty house does not, in itself, supply the means of sustenance; these have to be furnished by the wealthier man who employs the caretaker.

Another plea for vague sentiment in this matter is that we cannot expect that intelligent beings on other worlds would have the same form as man, and if not the same form, then, that the same conditions of existence would not hold good for them as for us. Both contentions are unsound. Protoplasm is the physical basis of all the life that we know, whatever its form; though these forms are to be counted by the million, and are as diverse as they are numerous. And everywhere and always, water is found essential to protoplasmic life. Of life of any other kind we do not know any examples; we have no instance; if such exist, then they are beyond our ken.

And neither anthropologist nor biologist would admit that the form of intelligent life was an unrelated accident. Whether the form brought the intelligence, or the intelligence the form, or both were evolved together, the one reacting on the other, the human form and the human intelligence are associated, and we feel this to be so of necessity. In 1891, Dr. Eugene Dubois found in Java a molar tooth and a portion of a skull, and later the thigh bone of the left leg, and two more teeth. Such as they were, these relics appeared nearer in form to the corresponding fragments of an average Australian than to those of an ape, and on this ground intelligence was claimed for the creature of which they were the remains, and it was given the name of Pithecanthropus, or Ape-Man. The discovery aroused much discussion, but on all sides it was unhesitatingly assumed that the difference between the form of Pithecanthropus and that of the most similar ape was an index of its superior intelligence over the ape, just in so far as that difference was in the direction of the modern human form. The same remark applies to the recent discovery of very ancient human remains in Sussex. Never at any time has it been supposed that the physical frame has followed any other path in the evolution of intelligence than that which brought forth man. The flesh-eating animals have attained efficiency in hunting and warfare by variation along many types of form; the herbivora have been not less varied in the forms by which as races they secured themselves from destruction; but Thought has been associated with the development of one type or form only, and the entire future of Thought on this planet rested neither with mammoth nor cave-bear, but with the possessor of the erect stature, the upward look, the differentiation of hand and foot, even in their crudest and earliest stages.