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.
INDEX
Alpha Centauri, 38, 42
Andromeda, 70, 85
Animalculæ, 165
Atmospheres, planetary, 105
Atoms and stars, 157
Aurora Borealis, 175
Bernoulli, 166
Bessel, 100
Bode, 188
Champollion, hieroglyphics, 143
Cherbourg, 97
Communication, interplanetary,
141
Earth currents, 135
Eiffel Tower, 164
Electrons, 137
Fontenelle, 184
Frauenhofer, 171
Galileo, 101
Giacobini, 97
God, 174
Gravitation, 105, 124
Gruithuisen, 100
Guzmann, prize, 153
Habitability of planets, 170
Harding, 101
Hercules, 160, 162
Homer, 95
Huygens, 185
Invisible universe, 175
Jupiter, 23, 41, 123, 125, 187;
(red spot), 126
Juvisy Observatory, 107, 135,
138
Kant, 182
Kircher, 184
Leibnitz, 166
Life, conditions, of, 106
Littrow, J. von, 141, 145
Magnetic variations, 131
Magnetic storms, 134
Mars, 14,105, 111, 145, 187, 204;
communication with, 117;
flashes on, 111; polar snows,
113; rotation, 114
Materialism, 173
Mayer, 101
Mercury, 185, 203
Milky Way, 163, 170
Molecules, 164
Moon, 9, 41, 105, 116, 141, 201
Neptune, 20, 35, 41, 219
Nice Observatory, 97
Pascal, 168
Renan, 88
Saturn, 18, 41, 189
Scheiner, 181, 192, 195, 210, 215,
220
Silicon, 210
Sirius, 47
Spectrum analysis, 127, 200
Stars and atoms, 157
Stars, speed of, 159
Sunspots, 134, 135
Temperature of planets, 127, 197
Venus, 95, 204; rotation, 107
Wavelengths of light, 171
THE END