Dreams of an astronomer

by Camille Flammarion

X. ARE OTHER PLANETS INHABITED?

CHAPTER X
ARE OTHER PLANETS INHABITED?

WE have a tendency to remain geometric and anthropocentric and to believe that everything is created on the terrestrial model. Not long ago, at one of the scientific soirées of the Solar Festival which I founded in 1904, and which is almost always illustrated by a conference of learned men of philosophical attainments, I requested my eminent friend Edmond Perrier, Member of the Institut and Director of the Paris Museum, to discuss the question of the population of the planets in the light of the latest achievements of science, in which he is past master. His reasoning was, that the same matter, the same forces, the same laws, exist on the Earth, Venus, Mars, Jupiter, etc., and that therefore the evolution of life is everywhere the same, arrested in one place and developed in another according to the conditions and circumstances, and that all organisms on all the planets can only have terrestrial forms. Our palæontology would be repeated everywhere.

It seems to me that this idea, shared, by the way, by other learned naturalists, is really too “naturalist,” too terrestrial, too classical, too professional, too narrow, too little in harmony with the grandeur of the universe, with the immensity of its energy and the variety of vital manifestations found on our small planet. On the contrary, the diversity of beings, already so prodigious on our little globe, must be, so to speak, infinite, and the extra-terrestrial living forms cannot be cast in the same mould. There is no reason, for instance, why all the beings of the universe should be limited to our five senses. There are inevitable differences: gravitation, density, food-supply, atmosphere, temperature, light, the years, seasons and days, etc., etc.—causes so different cannot fail to produce absolutely different effects.

A savant is a man accustomed to discussions and delighted to provoke them, because he knows that they contribute to the advancement of science; the naturalists whose views I dispute will not bear me any ill-will, and this is not the first time that they will pardon me for being a recalcitrant microbe.

The philosophers who teach that the universe is both infinite and homogeneous resemble microbes who think that their cell is the universe. Let us imagine the microbes in a particle of rust attempting to reason on the life of iron according to their own observations. For them, the world is a bit of iron attacked by oxygen. There is nothing else in nature. All their science leads them to conclude that the universe is made of iron. If they could have any vague notion about the existence of grass, insects, men, the sun, Jupiter or Sirius, they would be firmly convinced that they were all made of iron. If, in their particle of rust, they had observed the movements of translation and rotation in the constituent atoms of iron and could have risen to astronomical notions beyond their sphere, they would conclude that the other planetary systems are also made of iron and that none but iron-dwellers can exist. If we suppose that not only our own solar system but our visible universe and the other universes which succeed each other without end in the depths of infinite space are all constructed on the same zoological plan, we reason like these microbes.

Since the publication, a long time ago (1862), of my first work, entitled The Plurality of Inhabited Worlds, in which I expounded and discussed the conditions of habitability of the planets of our system as known, to science at the time of publication, several astronomers and philosophers have taken up the same question of the different aspects. Among these studies there is one which seems particularly worthy of attention, recently written by Professor Scheiner, Director of the Potsdam Observatory. I here offer my readers a condensed translation, asking them to excuse the Germanisms which render the style a little rough. But however bitter the rind, the fruit is good.

GENERAL REMARKS

“The aspect of the starry heavens during a clear and calm night gives a joy which only superior souls can feel. In the bosom of the general silence of nature and in the calm of all our senses our immortal soul whispers an undefinable language and forms conceptions which are difficult to express. If among the thinking beings of our planet there are vulgar spirits which remain slaves to vanity, this globe is to be pitied for having given birth to such creatures, but its value is enhanced by bearing on its surface intelligences capable of rising to the highest contemplations of the spectacle of nature.”

It is with these words that Kant in his Natural History of the Heavens terminates the last chapter, which treats of the habitability of the planets. They come from the soul of every thinking man who has preserved the spark of the ideal. The aspect of the starry sky awakens all those who are not among these vulgar spirits the same sensations which manifest themselves in various ways according to the education of him who experiences them and according to his momentary disposition. The astronomer also is delighted by this spectacle in spite of his regular observation of the sky. The astronomer sees farther and clearer into the celestial spaces than the ordinary man, his knowledge leads him by rapid deductions to vast considerations until he suddenly reaches a point where, for the time being at all events, an insuperable barrier presents itself to the human spirit, and where an imperious halt reminds him of the truth that all science is incomplete. The ordinary man looks upon the sky in quite a different way, and so does a woman. To both, the spectacle of the starry sky offers an enjoyment independent of all research and all preoccupation. A purely æsthetic pleasure.

Are these stars inhabited? Are they inhabited by thinking beings? Do love and hate reign there as they do here? Such are the questions which occur at once. Afterwards, in hours when the need for hope and consolation is felt, the desire arises to contemplate some day with our own eyes the splendours of those other worlds and to be able to soar up to them. The most brilliant stars are the special objects of such wishes, and if they could be fulfilled a population of loving souls would reside on these same stars: Sirius, Vega, Venus, and Jupiter.

The question of the population of the celestial bodies is as ancient as the discovery of the individual existence. It has always occupied thinking humanity, and many have sought to raise the veil which hides the answer. We, also, should like to take a step towards this solution, and it is with that object that we propose to quote here the actual facts bearing upon the conditions of habitability of the celestial bodies and to draw the most probable conclusions. We can combine the astronomical data with the most recent use of physics, and, thanks to the progress recorded by these two sciences in the last ten years, we may be able to draw some new conclusions.

(Here the author passes in review the older writers, Huygens, Kircher, and Fontenelle, which I have summarised in my two books called The Plurality of Inhabited Worlds and Imaginary Worlds. He arrives eventually at the nineteenth century and at Gruithuisen, whom my readers know less.)

HISTORICAL

Huygens, the most celebrated mathematician and physicist of his epoch (1629-95), takes it for granted that all the planets are inhabited (he does not speak of the Sun or stars), and that consequently all of them offer the conditions vital and essential to us, that is to say air and water. He easily refutes the objections based upon the distance of the planets from the Sun. He supposes that the water on the other planets will have quite other qualities than ours; on Mercury, for instance, it would only boil at a very high temperature, and on Saturn—at his time the outermost planet known—it would not freeze at the lowest temperature then conceivable. The mass of Jupiter compared with that of our own globe would suggest that its air must be very dense and that its inhabitants could swim in it; but the Jovians would easily accommodate themselves to this state of things. He supposed that the mind of the inhabitants of the other planets was about the same as ours and that their organism was analogous to our own; for what would be the object of the Sun illuminating those other planets if their inhabitants had no eyes? One might indeed think that different species of reasonable beings might exist, but not on the same planet, for they would be in mutual conflict, would struggle for supremacy, and do each other every kind of harm.

He gives a very naïve refutation of an opinion given before him that the height of the inhabitants would have to be inversely as the volume of the planet and consequently the men on Jupiter would not be bigger than the mice on the earth. This, he says, is not possible, because such small beings would not, as astronomers, be capable of using large telescopes.

An important question, according to Huygens, is whether the intelligence of the inhabitants has any relation to their distance from the Sun. He inclines to believe that the inhabitants of Mercury are much more intelligent than we on account of the greater force and vitality of their spirit due to the greater heat of the Sun, but this is not confirmed by what happens on our own globe. The same reason would lead one to believe that the inhabitants of Jupiter are much less intelligent than ourselves, although its four satellites would offer to the mind material for profitable astronomical studies.

What is also evident is the small value of a purely logical and philosophical reasoning based on insufficient premises. We see how a man of sense and judicious spirit can be led to absurd conclusions when he is burdened with preconceived ideas and incomplete knowledge. We shall have several more occasions to bring out this fact, and that is why we have avoided pretending to resolve the question we have put. The deductions which we formulate to-day may be reversed to-morrow by new discoveries furnished either by experience or by theory.

Apart from his decided opinion concerning the habitability of all the planets, Huygens draws his conclusions logically enough, inasmuch as he bases his affirmations on the knowledge of his time.

An entirely opposite method is used by the Jesuit Father Kircher.

He starts from the point of view that the principal object of nature is man and that all the rest is created for him. The planets are uninhabited because, apart from man, there could be no reasonable beings. But since they have upon man an influence determined by their astrological value, he finds the planets to be such as astrology represents them to be in their action upon us without considering their position with regard to the Sun.

On Mercury, everything takes place gaily and joyously, since all those who are born under its influence are inclined to lightness and mischief. On Venus, everything is even better—or worse: he finds everything gracious and charming; a soft rosy light is spread over the planet, perfumes are wafted about everywhere, zephyrs mingle their murmurings with those of the brooks, and gold and precious stones sparkle everywhere. Jupiter having, like Venus, a beneficent influence upon man, everything there is perfect: the air is pure and wholesome, the waters crystal-clear, and the soil itself as bright as silver. On Mars, on the other hand, everything is of a warlike roughness, forbidding and terrible, rivers of boiling pitch overflow their banks and envelop the country in thick and suffocating smoke. Saturn, as a planet, is particularly accursed; everything looks like a deserted grave. The planets are not inhabited by human beings, but by angels or genii who rule them.

(All these arguments of Kircher are not less infantile than those of Huygens, but they have left distinct traces in astrological literature, and even Victor Hugo reflects them eloquently.)

A contemporary of the two authors we have just mentioned is the Nestor of French writers, Fontenelle, who lived from 1657 to 1757, exactly a century. He described with much detail the inhabitants of the planets, and, like Huygens, he starts from the basis that they are all inhabited, and inhabited by beings formed according to the circumstances. On Mercury, according to him, the heat is so great that the rivers contain fused metals instead of water, particularly gold and silver; the inhabitants of this planet can therefore not imagine that there are worlds like the Earth, where gold and silver are solid and serve as money. Besides, the inhabitants of Mercury could not support the excessive heat if their planet was not animated by a movement of rotation so rapid that they are only exposed for a short time to the rays of the Sun. They are all inclined to be hot-heads and, like fools and infants, live without reflection and enjoy themselves in anticipation of the coolness of the night. Littrow remarks on this subject that Bode, the translator of Fontenelle and at one time Director of the Berlin Observatory, is seriously astonished by this opinion of Fontenelle and exclaims: “Very strange! for with us in Berlin we find that a great heat makes people lazy and sleepy instead of lively and active.”

The inhabitants of Venus only render homage to the goddess of love. They are not interested in philosophy or mathematics, read no books or journals, pass the whole day in their flirtations, and practise in a superior manner the arts which appertain to them, music, poetry, dance, etc., but they are not adept at cookery, for they live almost entirely on air. They are not beautiful, but their amorous character prevents them being influenced by their ugliness; they are Celadons and Sylvanders. Wieland certainly did not know the works of Fontenelle, or he would probably have located one of his romances or his love-stories on Venus as depicted by that author.

Fontenelle’s procedure with regard to Mars is rather singular. He declares that that planet does not merit any attention. Our imaginative savant hardly wants to say anything about Jupiter either. He gives a description of the aspect offered by the whole solar system as seen from that planet. He explains how Venus and Mercury are invisible there without the aid of a telescope, and that the Earth only appears as a point. The volume of Jupiter causes him some embarrassment, for whereas the inhabitants of Mercury on account of the small dimensions of that planet nearly all know each other, those of Jupiter cannot possibly do so.

On account of the extreme cold, the life on Saturn is still more disagreeable than that on Jupiter. If the Saturnians were brought to the Earth they would certainly die of heat even in Lapland. If the water on Saturn is of the same nature as ours, it must look like our polished stones, and spirits of wine must resemble diamond. In consequence the inhabitants of Saturn cannot but be slow and phlegmatic; they know no gaiety and remain like oysters in the place they were born in.

Fontenelle continues in this way without attaining any depth, under the impression that our planet is the type of the universe. Let us now pass to the nineteenth century.

Graithuisen, the Director of the Munich Observatory, published his chief works in the first thirty years of this century. His researches relative to the habitability of the planets were, therefore, made at an epoch when we already possessed important data on their physical constitution, an epoch when, thanks to the work of Bessel, the golden period of astronomy had already begun. One may therefore take it that the work of Graithuisen marks a real progress beyond his predecessors, as indeed he says himself without ceremony. But as a matter of fact he is rather fruitless, as is shown by the strange manner in which he deals with the earth-light on Venus.

He already knew of the phenomenon, which is insufficiently explained even in our day, that during the phases of greatest visibility of Venus the dark side appears to have a faint luminosity. “The simplest explanation to give of this,” he says, “is that at the epochs when this faint light on Venus is visible, the inhabitants of the planet organise festivals and general illuminations, which are the easier to arrange on account of the vegetations of Venus being incomparably more luxuriant than even the virgin forests of Brazil. These festivals are probably celebrated on the occasion of political changes or according to religious periods. Now, the principal observations of the ashen light on Venus are those of Mayer in 1759 and of Harding in 1806.” Whence he draws the following conclusion: “Between the observation of Mayer and that of Harding 76 years of Venus and 47 Earth years have elapsed. If this period has a religious character, we cannot see a justification for that number of years, but it becomes more comprehensible if we assume that some Alexander or Napoleon then attained universal power. If we assume that the ordinary life of an inhabitant of Venus lasts 130 years of Venus, which amount to 80 terrestrial years, the reign of such an autocrat could easily last 76 years as reckoned on Venus. I have no intention to press this opinion and do not claim its credibility, even should it appeal to the reader’s imagination; but if my hypothesis is correct, we at least receive direct testimony to the existence of inhabitants on Venus. Even if the period were shorter, the phenomenon might still be due to some other observance. One could celebrate all the great festivals by similar illuminations which would sometimes follow close upon one another. The fires would serve another purpose, inasmuch as they would thin out the forests and provide fresh arable ground for an increasing population. Large migrations of people would be prevented, and the consequent wars would be avoided and the race would remain united.”

One must acknowledge that these ideas of Gruithuisen are most fantastic. I have frequently observed this unilluminated hemisphere of Venus, notably in September 1895 and in April 1897: it appeared to me of a violet colour, and the idea of illuminations by the inhabitants is pure romance.

He also then passes in review the ideas of Kant which one finds expounded in the two works cited above and which therefore I need not quote. We know that for the Philosopher of Königsberg the intelligence and the degree of perfection of the inhabitants of the planets is in proportion to their distance from the sun.

We may add to the remarks of M. Scheiner that the writers who have dealt with the question of the plurality of worlds have nearly all judged the planets from the appearances they present to us from our point of observation, and have assumed the harmony of nature according to the manner of Bernardin de St. Pierre. For this occasion I have re-read his hook with some interest. To this very simple member of the Institut, Venus is a bright world peopled by amorous natives “who give themselves up to dance, festivals, and songs, or compete for swimming prizes, like the happy islands of Tahiti”; the inhabitants of Mars are warlike, “resembling the northern Germans, their forests and hills, their atmosphere resounding to the warlike sound of their horns and that of drums and trumpets which announce the spilling of blood”; on Jupiter “they resemble the Dutch, being industrious, patient, wise, reflective, and tending their numerous herds in their vast fields,” etc., etc.

The author of Paul et Virginie remains purely terrestrial in these descriptions, which he believes to be astronomical.

But let us return to the dissertation of M. Scheiner. He passes on to the purely scientific aspect of the question, which is the only one which interests us here.

THE ORIGIN OF LIFE

The problem of knowing if worlds other than the Earth, being habitable, are really inhabited, depends really upon this question: How did life appear on this Earth?

It is irrefutable that there was a time when the Earth was not habitable in the usual sense. Therefore life necessarily had a commencement. “This can have taken place in three different ways, either by a special act of creation, in which case it is of little importance to our problem to know whether this act was accomplished in a complete manner as taught by the Bible or whether it was limited to the creation of the inferior forms of life; or by spontaneous generation; or, lastly, by the importation from space, in which we can just as well imagine germs of life as gases and inorganic substances.

“From the philosophical point of view, these three hypotheses are equally well-founded, for none is more easily conceived than the other and observation has not yet confirmed any of them.

“If we admit the first hypothesis, a creative act, such a manifestation of the will of a Supreme and Impenetrable Being is beyond the laws of nature. In this case we have no means of reasoning on the purpose of the Divine Will. We do not know if the creative act has been exercised duly once in favour of the Earth, or whether it is renewed on various occasions, or whether it takes place in the same manner every time a heavenly body becomes fit to act as an abode of living beings. We cannot express any opinion on that and therefore our initial question remains open. In the narrow Biblical sense the Earth and mankind must be considered the last word in creation and the idea that other reasonable beings might exist is necessarily eliminated.” It seems to me that in our times none of the readers of this book need consider the first hypothesis as admissible. Everything shows us that living beings have not been directly created by a Supernatural Will, but that they have slowly and gradually evolved during the geological periods which are known to us. Scheiner then passes on to the second hypothesis. “By spontaneous generation,” he says, “is meant the formation by material molecules of an organism of the most rudimentary species, which involves our attributing to an inorganic substance properties which end in the production of life.

“The adoption of these properties is possible in two ways: by a sudden coincidence of favourable circumstances, or by a continuous process which, thanks to a gradual development, fills up the gap of continuity which at present appears to exist between inert matter and living matter.

“If circumstances on our earth have favoured spontaneous generation, there is no reason why they should not have done the same on other celestial bodies having a similar constitution, and one may deduce with certainty that all the heavenly bodies which are in this condition are provided with similar organisms.” The author then examines the third hypothesis.

Space can be filled with organised matter or matter capable of life distributed almost uniformly, without being specially destined for any particular body, since we must assume a commencement for each. The surface of the heavenly bodies receives the organisable matter which, when its finds the necessary conditions, develops and forms living beings. It is clear that in this case the presence of organised beings on all the heavenly bodies capable of entertaining life is not merely a probability but a certainty.

Scheiner considers the three hypotheses as equally acceptable. The first, he says, is a matter of sentiment. It does not solve the question, since it would involve a Divine Will. Out of the three hypotheses the last two would solve our problem in an absolutely affirmative manner, whereas the first leaves it undecided.

“We only wish to prove one thing by our argument, and that is that the opinion that habitable heavenly bodies are really inhabited is much more probable than the contrary opinion, and this authorises us to continue to develop our thesis.”

THE CONDITIONS OF LIFE

The author here puts a fundamental question. What is life and what is living matter! Libraries have been written on this subject but all to no purpose, for we are as unable to seize the essence of life and of living matter as that of gravitation, for instance, though the latter appears to us infinitely simpler in its manifestations than what has been called vital force.

We only know that on our Earth vital force is united to a special form of matter called organised matter, and that when this organised matter disappears it ceases to exist as vital force and transforms itself—since energy is indestructible—into other forms of energy. It is not of material importance whether vital force is regarded as a special force or a special aspect of a known force, e.g. electricity. It follows from this intimate association of vital force with living matter that the vital manifestations (nutrition, growth, reproduction, etc.) can only take place under conditions in which living matter can exist; in all other cases the manifestations of life cease and death ensues, or the vital force becomes latent until favourable conditions return.

Our problem of the habitability of heavenly bodies is therefore limited to the question of celestial bodies on which conditions are such that living matter can exist in a permanent form. We must therefore enquire first of all what those conditions are, after which we shall be able to use our astronomical resources to find whether these conditions are represented on the other centres of condensation of matter in the universe.

The conditions necessary to life are the more numerous the more complicated the structure of organic matter. The maximum of requirements is therefore attained in the higher animals and in man.

The simpler the organism, the simpler are its conditions of existence and the greater are in general the possibilities of supporting unfavourable conditions.

Animals and plants living in caves or at great depths under water are deprived of light; they have accommodated themselves to that privation and do not suffer by it. Animals require oxygen in air and in water; plants also require a small quantity of carbonic acid for building up their tissues. There are even animalcule in existence for whom oxygen is a poison. As a general rule, temperatures above 50 degrees centigrade are insupportable.

This is due to the fact that at that temperature albumen, one of the most important substances in the animal organism, coagulates. Inferior beings can resist higher temperatures and even for a short time 100 degrees centigrade, which is the boiling-point of water, but they could not live long. In its liquid form water is indispensable to organic life; life of any duration below zero is impossible because the water contained in every organism would solidify and the parts composing the organism would lose their mobility. Lack of water does, however, not inevitably imply death, and plants particularly can preserve for a long time a latent vitality while deprived of water. Cereals furnish a striking example, for when dried they can preserve their germinating power for years. Although living matter can preserve its vitality so long, it is none the less true that during that period all manifestations of life cease. If therefore the lack of water is perpetual, life must be considered as really suppressed.

Even for the lowest forms of life three conditions must be regarded as essential: water, an atmosphere containing oxygen and carbonic acid, and a temperature between the limits indicated above.

It is therefore really from these three points of view that we must study the heavenly bodies in order to be in a position to judge whether organic life as we know it is possible on them or not. As regards knowing whether that life presents itself under forms comparable to those which we see here, whether there are, for instance, beings analogous to humanity, that is quite another question.

The means at the disposal of astronomy for determining the constitution of the heavenly bodies are of various kinds. We can take into account phenomena which at first sight do not seem adapted to that end.

Direct observation with the help of the telescope enables us to discover the surface details of the planets and any changes which take place in them. Such changes in most cases imply the existence of an atmosphere. Observations of occultations of stars by the Moon or the planets lead to the same result. Theoretical astronomy tells us the distance between the planets and the Sun; physics tell us the quantity of light received by each planet from the Sun; and the period of revolution and the inclination of the planet’s axis tell us about the course of its seasons. Photometry gives us the amount of sunlight reflected by the surface of the planet, and thus furnishes indications concerning certain properties of the planetary surface, properties which permit us to decide with certainty, for instance, whether light is reflected by a solid surface such as the ground or whether the rays do not penetrate so far and are sent back in the upper parts of the atmosphere by lays of clouds.

It is spectrum analysis which furnishes, as we know, the most important auxiliary information; it presents the heavenly bodies to the eye of the mind as the microscope unveils to the eye of the body the marvels of the infinitely small. The rays of light are messengers who, having passed through the spectroscope, bring to us news of the most distant worlds and tell us of the temperature of the fixed stars, of the metals volatilised in their atmospheres, of the incredibly low temperature of the nebulæ and of the gases which envelop the planets.

We do not here wish to intone a hymn to spectrum analysis; we only wish to report briefly and simply what we know of the physical nature of the celestial bodies. But we must admit that the greatest amount of that knowledge is due to the spectroscope.

THE PLANETS OF OUR SYSTEM

(We continue the translation of the essay of the Potsdam astronomer.)

In the light of contemporary astronomical knowledge let us make a rapid survey of the other worlds.

The Moon

Our lady readers will not raise any objection if we occupy ourselves at first with the Moon, this confidante of every heart, either happy or unhappy. The most important thing about her for the moment can be put into one sentence: she has neither air nor water, and her temperature oscillates between extremes separated by more than 200 degrees centigrade. None of the conditions stated above is therefore fulfilled, and accordingly no organic life could exist on her. It is also interesting to see how the cessation on the Moon of one of these vital conditions has entailed the cessation of the two others. There is no occasion to doubt that the Moon formerly possessed an atmosphere; by analogy with the planets this must indeed be assumed as altogether certain. The feeble mass of our satellite which, on the one hand, has been the cause of its rapid cooling, has, on the other hand, brought about the dissipation into space of its atmosphere, which was probably always of small density.

But the smaller the pressure of the air the more rapid is the evaporation of water, and this is why its disappearance coincides with that of the atmosphere. Besides, the complete absence of air allows the rays of the Sun to penetrate without hindrance to the ground and to heat it to a high temperature during the 14 times 24 hours of duration of the lunar day. During the night, which is of equal duration, there is radiation of heat into celestial space, and the soil cools down to a temperature which cannot be very different from the absolute zero of the temperature of empty space.

(I cannot entirely accept these allegations with regard to the habitability of our satellite. The absence of air and even of water is pot proved. The variations actually observed even prove that the moon is not altogether a dead world. But let the author continue.)

Such a fate is also in store for the Earth, and nothing can save it. Our Earth will also one day become a barren body, incapable of supporting organic life, a deserted grave of the civilisation created by the human spirit. Just as the isolated individual disappears, so will humanity one day disappear entirely. In a limited domain death is always finally victorious; but, on the other hand, a new life flourishes elsewhere, and when that new life is developed, some day perhaps on another planet of our system a scientific article will be written on the question whether the Earth is still habitable.

Mercury

Our knowledge of the physical constitution of Mercury is very slight. It seems to be surrounded by a light atmosphere which contains water-vapour. Since the solar heat on Mercury is about seven times stronger than on us, the extreme limits of temperature above indicated must here be very considerably surpassed, and water can hardly exist except in the form of vapour. But the argument is entirely changed if we accept the recent discovery of Schiaparelli, according to which the duration of rotation of Mercury would be identical with its period of revolution, so that, like the Moon to the Earth, Mercury would always turn the same face to the Sun. On that side the temperature would naturally be very high, while the most intense frost would reign on the other side. But between these two extremes there ought to be a mixed zone in which our three conditions might possibly be realised, so that we may assume for Mercury a limited habitability.

Venus

On Venus the Sun’s heat is still very considerable, and in the torrid zone it would be insupportable to us. But, on the other hand, this planet is surrounded by a dense atmosphere, which on account of the presence of water-vapour proves the existence of water on the planet. The higher regions of the atmosphere are occupied by a thick layer of clouds which hardly ever allows our gaze to reach the ground, but which is equally opposed to the passage of the Sun’s rays. Much more than half the solar radiation is reflected by that layer of cloud, and we can suppose, on the whole, that the upper limit of a tolerable temperature is not passed at the surface of this planet. And as we have already said, since there is water and an atmosphere, we have no reason to doubt the habitability of Venus.

Mars

As regards Mars, which commences the outer series of planets, we obtain a still more satisfactory result. We can clearly recognise the subdivision of the surface into water and dry land. Its atmosphere has properties which agree with those of our atmosphere. Not only is there certain evidence of the existence of water-vapour, but spectroscopic studies have proved also that the principal components of the atmosphere are the same as those of the Earth’s atmosphere—that is to say that there is oxygen and nitrogen. Sometimes the soil is hidden by groups of clouds, sometimes they disappear to appear again in other places. Its poles are encased in snow and ice, the white area of which varies in extent according to the seasons. Besides a number of enigmatical facts—we need only refer to the canals and their doubling—there are numerous meteorological phenomena on Mars which frequently occur in our own atmosphere. Although the temperature of Mars is, on account of its farther distance from the Sun, sensibly below that of the Earth, the difference is not sufficiently considerable to oppose a serious obstacle to the stable existence of organised matter in the torrid or temperate zone. The torrid zone of Mars must correspond approximately in climate to our temperate zone. We can therefore finally declare with entire conviction that the conditions offered by Mars are suitable for life as we know it on Earth.

Jupiter, Saturn, Uranus, and Neptune

With the planets Jupiter, Saturn, Uranus, and Neptune we enter a region quite different from the preceding one. These bodies all possess atmospheres of great density in which, on Jupiter for instance, we can observe immense revolutions and violent cataclysms. They also contain water-vapour, but besides that they contain a substance characterised by a strong absorption of certain red rays. This gas is found in a small quantity on Jupiter, but in much greater quantities on Saturn and Uranus. On the latter planet the atmosphere, apart from the water-vapour contained in it, does not seem to have any analogy with ours; strong absorption-bands show in the less refrangible portions of the spectrum. As we have seen that oxygen is not absolutely necessary to organic life, the single fact that the outer planets have an atmosphere may suffice to show that our first condition is fulfilled. The presence of water-vapour in their atmosphere proves the existence of water and fulfils the second condition. As regards the third condition, Jupiter also seems to satisfy that, at least in equatorial regions, especially if we take into account that radiation of heat into space is much limited by the thick atmosphere filled with clouds. The farther we go away from the Sun the more does the third condition become precarious, and while we may have some doubt concerning Saturn, it cannot be denied that on Uranus and Neptune the solar heat is insufficient to support organised life in a durable way.

* * * * *

(The atmospheres might be formed of gases which would make the radiation almost zero and would produce a relatively high temperature. This certainly happens in the case of Mars, whose temperature is not lower than that of our globe, where the polar snows are less dense than ours and melt more completely in the course of the summer.)

* * * * *

But another peculiarity presents itself which may reverse all our ideas relative to the planets from Jupiter outwards. Certain observations tend to show that Jupiter is not yet cooled down, that its real nucleus is still fused or perhaps even in the gaseous state, and that it has not yet formed a solid crust upon which life might develop. Besides the phenomena directly observed or revealed in the spectroscope, the very small specific gravity of these planets also supports that hypothesis—the density of Saturn is about the same as that of cork. It is very difficult to form an exact idea of the constitution of these planets. On the other hand, it is possible to believe that one of the extreme planets, Uranus for instance, is sufficiently cooled to possess at least a liquid surface which, on account of the internal heat of the planet, may have preserved for a certain period a temperature sufficient to entertain life even after the solar heat is no longer sufficiently powerful But these are only hypotheses.

* * * * *

Let us summarise in a few words the results of this chapter.

On the Moon no organic matter can exist; we may suppose living beings to exist in a small zone of Mercury; the surface of Venus is very probably habitable in most of its regions; Mars is certainly habitable and probably under such conditions that certain species of our plants and animals could, if transported to that planet, continue to live there.

For the other planets the possibility of habitation cannot be entirely denied, but the existence of living beings on their soil is not probable. Let us add, for the sake of completeness, that on the Sun, which radiates the most intense heat, and on those innumerable other suns which we call stars and which the telescope reveals to us, living matter certainly does not exist.

It follows that of the millions of stars visible to us in the universe there are only two or three which we could consider with any certainty capable of being inhabited as we conceive it. That seems a rather unsatisfying result, which allows an icy sentiment of loneliness in infinite space to take possession of our souls.

We have seen in the first part of our work how the speculations of those who formerly discussed the habitability of the planets not only raised the question whether such and such a planet was inhabited, but also tackled the much vaster problem of the particular characters of the beings which lived there. After attentively following the discussion, nobody should ask to penetrate further into this question. It has indeed been a great effort to obtain a positive result at all, even while restricting the population of the planets to the simplest forms of organised matter. The number of forms under which it shows itself on the Earth is so considerable that we can only be dazed by its abundance, and this impression must be even increased if we remember that the possible forms are far from being exhausted and that we only know those which, satisfying terrestrial conditions, have been able to survive by adapting themselves to their surroundings. We do not exaggerate in declaring that Nature recognises no limit to the number of forms which can harbour life, and this observation is an argument in favour of those who desire to find reasonable or superior beings in distant worlds. It indicates that as soon as the first germs of life exist, the possibility of a complete development is there, and that however different the external conditions may be, Nature is not at a loss for varieties of forms of life. That is why we have the right to hope that on Mars, for instance, there are beings who not only show manifestations of animal life, but who are endowed with intellectual faculties. That, however, is all we can acknowledge: the right to hope and not to a certainty.

Yet at the risk of exposing ourselves to a criticism similar to that which we have in the first part directed against other authors, we would now invite the reader to follow us for a moment into the domain of speculation and hypothesis.

THE POSSIBILITY OF BEINGS OF DIFFERENT CHEMICAL CONSTITUTION

In this chapter Scheiner develops the idea indicated above (page 127) on the possibility of the existence in other worlds of living beings entirely different from ourselves.

We have up to now (he writes) understood by organised matter something of which carbon, combined with hydrogen, nitrogen, and other elements, is the principal chemical component. Carbon is the essential constituent; organic chemistry is the chemistry of carbon. We do not know any other substance which would allow of such an array of combinations, but the possibility of such a body cannot be denied. While on Earth all life is bound up with carbon compounds, one may suppose that in entirely different circumstances another element might show itself capable of supporting the conditions of life in combinations which might resist greater heat without decomposition or greater cold without becoming torpid. A few years ago we seemed to be on the track of something like that. Silicon is the element which has the greatest chemical analogy to carbon, and in combination with oxygen it is found in enormous quantities in the form of silica, and all its combinations have very characteristic properties. Just as in organic matter every being forms itself into cells by fission or conjugation, so also can we produce from combinations of silicic acid a cell from which under our eyes an object develops which has a vegetable appearance. This experiment, easy to make, is perhaps unknown to many of our readers, and therefore we must say a little more about it.

Most combinations of silicic acid with metals are insoluble in water. Some, however, and especially a combination with potassium known by the name of water-glass, occurs in a soluble form. If into such a solution we introduce a small quantity of any soluble metallic salt such as chloride of copper or silver nitrate, decomposition takes place and the chloride of copper is converted into silicate of copper and chloride of potash. A very curious phenomenon then takes place. At the moment when the first traces of the metallic salt are dissolved, the decomposition mentioned above takes place and the molecule of salt is covered with an extremely fine skin of insoluble silicate. It is across this skin that the liquids are then exchanged by a process of osmosis which, as we know, plays a great part in the life of plants and animals and which here takes place as follows:

More liquid enters the membrane than issues from it; consequently the internal pressure increases until it is great enough to break through the membrane, somewhere allowing a drop of the liquid which is a solution of metallic salt to escape. On account of the chemical decomposition, this drop immediately surrounds itself with a new skin, and the process of rupture and reformation is repeated until the metallic salt inside the membrane is completely decomposed. We thus see arising under our eyes in a few minutes a marvellous arborescent structure which could at first sight be taken for some plant of inferior order. The rupture of the membrane always takes place in the most recently formed portion, because there the membrane, which thickens gradually, has as yet the least thickness; that is generally the upper portion, so that this offers another analogy to plant life. The coloration of the cells differs with the nature of the metallic salt, but we cannot trace any influence upon the forms obtained. By throwing into the solution several different metallic salts, one can form a many-coloured garden in a small bottle.

We are far from seeing in this experience anything but a quite external resemblance to the phenomena of living matter. Yet it leads us to other reflections, especially as we do not yet know anything concerning the real nature of living matter.

Among plants and inferior species of the animal world we cannot imagine a life conscious of itself. But in common with our silica creature they show growth and the consumption of chemical substances. In both cases that growth is arrested wherever and whenever the food gives out. This happens in the silica solution when the provision of metallic salt is exhausted; and these are not the only analogies which one can find.

But it will be said that there is in reality a capital difference: in one case we have to do with real life for which we have no explanation; in the other we have to do with a simple chemico-physical phenomenon.

We reply that a few centuries ago this chemical vegetation would have been an enigma as life itself is still to us, and nobody would then have doubted that he was observing the development of some strange plant. Would they not then have taken the silicon cell for an organic cell? What follows? That the idea of living nature is quite relative, that it changes with our knowledge, and that an imaginative spirit is quite at liberty to endow the stare which we just excluded from life with a life quite different from ours.

Apart from these possibilities, the somewhat discouraging result of our previous study was that of all celestial bodies visible to us only two or three could be described with any probability as fit to support something resembling our terrestrial organic life. We cannot object to this conclusion so long as we take it literally, but we wish to point out that all depends upon the little restriction contained in the words “visible to us.”

We must therefore include the heavenly bodies which we cannot see and concerning which we know very little. But here we encounter a very peculiar paradox. We know nothing about these invisible stars, and yet as far as our interests are concerned we know more about them than the others. For mathematics comes to our aid, and if we rely on the calculus of probabilities we arrive, as we shall see, at very clear results.

Our Sun has created for itself, without counting the asteroids, a retinue of eight planets, which on account of their respective distances from the central body are placed in the most diverse conditions as regards temperature. Out of these eight planets one, the Earth, is undoubtedly inhabited, and two others, Mars and Venus, very probably. From the fact that the Sun has produced not one planet but eight, we can conclude that very probably the other suns or fixed stars have also produced one or several planets, and that those which escape from that law are the exception. We must also admit that among these stellar planets there may be some in such a condition and at such a distance from their central sun that organic life is possible on their surfaces.

We shall make this calculation with figures so modest that we shall obtain results obviously below the truth.

The number of stars revealed by a telescope of moderate power amounts to 10 millions. If we suppose that every star only has on an average one planet, we obtain already the considerable number of 10 million planets. It is true that with us three planets out of eight may be considered habitable, but let us suppose the proportion in the universe is only one in a hundred. We shall still have no less than a hundred thousand habitable planets.

This number, evidently falling short of reality, makes already quite another figure compared with the three habitable worlds we had before.

After that the universe no longer appears such a desert. There is nothing to hinder us from giving rein to our fancy and imagining on the one hand the strangest forms of life among the innumerable planets gravitating round the stars, and on the other hand thinking beings which surpass us considerably in intelligence and to whom our most difficult problems are as transparent as self-evident truths. The conclusion is that in all sorts of degrees we must see Life radiating into space and lighting up Infinity.

REMARKS

To this interesting study of Monsieur Scheiner we may add in the first instance that the question is still more complex, that we can consider the resources of Nature as infinite, and that “positive” science founded only upon our senses is quite insufficient, though it may be the only basis of our reasoning. It is through the eyes of the spirit that we must survey the universe.

As we have seen, this new examination of the question of the habitability of other worlds by thinking beings presents a new interest, and the author has been able to escape the usual error of scientific writers, which consists in supposing that the first condition of habitability of another world is that it resembles the Earth. That is always the reasoning of the fish, which would affirm without an absolutely logical conviction irrefutable to himself that it is impossible to live outside water. But it seems to us that our conception of the universe can be even more vast and elevated than that of the learned German astronomer.

As regards the planetary systems differing from ours, we are no longer obliged to fall back upon suppositions. We know already with certainty that our Sun is no exception, as some theoreticians maintained even quite recently. The discovery is of considerable interest.

It is surely a rather exceptional situation that a sidereal system consisting of a central sun and one or more bodies gravitating round it should have its system just in our line of vision so that the revolution of the bodies which compose it should bring dark bodies between us and the star and produce a more or less complete eclipse. Since, on the other hand, such eclipses would be our only means of proving the existence of these unknown planets (except perturbations as in the case of Sirius and Procyon), it seems that it would have been absolutely daring to hope for such a circumstance to discover solar systems different from ours. Yet this exceptional case occurs in various parts of the sky. Thus, for example, the variable star Algol owes its variation of brightness, which reduces it from the second to the fourth magnitude at intervals of 69 hours, to the interposition of a body between it and the Earth, and celestial mechanics has already been able to determine with precision the orbit of this body, its dimensions and its mass, and even the ellipticity of the Algol sun. Thus we have here a system of which we know the sun and one enormous planet whose revolution takes place in 69 hours with a very high velocity as measured in the spectroscope, a planet still self-luminous, although less luminous than its sun, as the Earth was long ago, but a planet in the course of cooling and approximating to the state of Jupiter.

The star Delta Cephei is in the same case: it is an eclipsing variable with a period of 129 hours, and its eclipsing planet also revolves in the plane containing our line of vision. The star U Ophinchi shows a similar system, and observation has revealed several others.

If therefore chance has brought it about that a certain number of different solar systems should have been revealed to terrestrial observation by presenting side views, that is evidence of the existence of innumerable solar systems disseminated through the depths of space, and we are no longer reduced to mere conjectures.

On the other hand, the analysis of the movements of several stars, such as Sirius, Procyon, Altair, and many others, proves that these far-distant suns have companion planets as yet unrevealed by the telescope, and which possibly may never be discovered because they are dark and lost in the radiation of the star. The companion discovered in the neighbourhood of Sirius is not the only celestial body of that system. Scheiner speaks of about 10 million stars as constituting the sidereal universe. But the photographic chart of the heavens which comprises stars down to the thirteenth magnitude is already expected to contain 30 millions. If we go down to the lowest magnitudes we reach the figure of 100 millions. It is therefore not an army of 100,000 worlds which appears before us, but rather of several millions.

Now, this is a point of the greatest importance for the exact appreciation of the problem.

The terrestrial organisms from the lowest up to man are the result of forces in action on the surface of our planet. The first organisms seem to have been produced by combinations of carbon with hydrogen and oxygen, and their life consisted, so to speak, only in a few rudimentary sensibilities. Sponges, corals, polypi, medusæ, give us an idea of these primitive beings. They were formed in the warm waters of the primary ages. While there were yet no continents nor islands emerging from the universal ocean, there were no air-breathing organisms. The first aquatic beings were succeeded by amphibia and reptiles. Afterwards came the mammalia and the birds. The constitution of beings stands in close relation with the substances of which they are composed, the medium in which they live, the temperature, the light, the density, gravitation, length of day and night, seasons, etc.—in a word, all the cosmographic elements of the planet.

If, for example, we compare two such worlds as the Earth and Neptune, which differ very much as regards distance from the Sun, we cannot imagine for a single instant that the organic forms should have had the same development. The average temperature must be much lower on Neptune than on the Earth, and so must the intensity of light. The years and the seasons are 165 times longer than with us; the density of materials is three times less, and gravitation, on the other hand, is a little stronger. Under conditions so different from ours, the activities of Nature can only have been shown in other forms. The elements also are not found present in the same proportions, and spectrum analysis has even shown us that substances which prevail in the atmospheres of Jupiter, Saturn, and Uranus, as well as Neptune, are different from those which constitute our organisms. Lungs functioning in another atmosphere would have to be different from ours. The same applies to the stomach and the digestive organs. Chemical constitution is not even the same. Instead of carbon as a fundamental element associated with hydrogen, oxygen, and nitrogen, we can imagine with Scheiner silicon and other bodies. We must conclude that the organs and the senses cannot be the same as they are here. The optic nerve, for instance, which has been formed and developed here from the rudimentary organ of the trilobite to the marvels of the human eye, must on Neptune be incomparably more sensitive than it is in our blinding sunlight and must perceive radiations which we do not perceive here. It may even be replaced by another organ. The bodily forms, animal and human, can resemble nothing on earth.

Certain savants object that, if the conditions are too different from terrestrial conditions, life cannot exist at all. But we have no right to limit the powers of Nature by the narrowness of our sphere of observation, or to pretend that our planet and our human race are the model for all planets. That is an hypothesis as infantile as it is ridiculous.

Others go still farther, and imagine that life only appears on Earth, and that we have no sufficient reason to suppose that on other globes it has been the result of inorganic evolution. This, as we have often repeated, would be a strange interpretation of the language of Nature, considering that our small planet is too small a cup to contain the whole of life, that that life abounds everywhere, fills the waters, swarms in the air, covers the entire surface of the globe, and that the fertility of Nature is such that she multiplies parasitic life at the expense of life itself, rather than get tired of producing. And the spectacle is the same for all the immense duration of the geological eras. Quite lately, noticing a heap of fossils of the secondary era by the roadside in the country, I took a stone to put it into a collection, for it was entirely made up of shells petrified and cemented in a block. In taking it up I exposed a swarming mass of living beings, small snails, wood-lice, beetles; and I caught two lizards; while butterflies laid their eggs in the plants around. Life of former days, life of to-day, life everywhere. Life always!

Certain minds are capable of supposing that for the whole duration of its existence, for millions of years, a world could come to nothing but the state of dead and barren rock and that the life which swarms on the surface of our planet is only a freak due to the fortuitous combination of elements of fruitfulness, a parasitism of more or less large fleas which might never have been produced at all. But that is a hypothesis contrary to the observation of Nature, and difficult to maintain seriously except as a pure play of the imagination, which does not satisfy the most elementary logic. And though our logic may not be that of Nature, yet we must not stray too far away from it if we want to reason.

But as we have already repeated so often in this book, and what we must thoroughly absorb, is the importance of time as well as of space. Just as our world is nothing but a small island, a point in the universe, so also our era is nothing but a moment in eternity. The present moment has no more importance than the moments which have preceded or those which are to come. There is no reason to believe that such and such other worlds are inhabited at present simply because we live at present and can observe them. One world has been inhabited in the past, another will be inhabited in the future. Let us not be personal, like infants or the aged, who see only their own room. Let us know how to live in the infinite and in the eternal.