the Planets

Although it has been admitted by the late Mr. Proctor and some other astronomers that most of the planets are not now habitable, yet, it is often urged, they may have been so in the past or may become so in the future. Some are now too hot, others are now too cold; some have now no water, others have too much; but all go through their appointed series of stages, and during some of these stages life may be or may have been possible. This argument, although vague, will appeal to some readers, and it may, therefore, be necessary to reply to it. This is the more necessary as it is still made use of by astronomers. In a criticism of my article in The Fortnightly Review, M. Camille Flammarion, of the Paris Observatory, dramatically remarks: 'Yes, life is universal, and eternal, for time is one of its factors. Yesterday the moon, to-day the earth, to-morrow Jupiter. In space there are both cradles and tombs.'[19]

It is thus suggested that the moon was once inhabited and that Jupiter will be inhabited in some remote future; but no attempt is made to deal with the essential physical conditions of these very diverse objects, rendering them not only now, but always, unfitted to develop and to maintain terrestrial or aerial life. This vague supposition—it can hardly be termed an argument—as regards past or future adaptability for life, of all the planets and some of the satellites in the solar system, is, however, rendered invalid by an equally general objection to which its upholders appear never to have given a moment's consideration; and as it is an objection which still further enforces the view as to the unique position of the earth in the solar system, it will be well to submit it to the judgment of our readers.

Limitation of the Sun's Heat

It is well known that there is, and has been for nearly half a century, a profound difference of opinion between geologists and physicists as to the actual or possible duration in years of life upon the earth. The geologists, being greatly impressed with the vast results produced by the slow processes of the wearing away of the rocks and the deposit of the material in seas or lakes, to be again upheaved to form dry land, and to be again carved out by rain and wind, by heat and cold, by snow and ice, into hills and valleys and grand mountain ranges; and further, by the fact that the highest mountains in every part of the globe very often exhibit on their loftiest summits stratified rocks which contain marine organisms, and were therefore originally laid down beneath the sea; and, yet again, by the fact that the loftiest mountains are often the most recent, and that these grand features of the earth's surface are but the latest examples of the action of forces that have been at work throughout all geological time—studying all their lives the detailed evidences of all these changes, have come to the conclusion that they imply enormous periods only to be measured by scores or hundreds of millions of years.

And the collateral study of fossil remains in the long series of rock-formations enforces this view. In the whole epoch of human history, and far back into prehistoric times during which man existed on the earth, although several animals have become extinct, yet there is no proof that any new one has been developed. But this human era, so far as yet known, going back certainly to the glacial epoch and almost certainly to pre-glacial times, cannot be estimated at less than a million, some think even several million years; and as there have certainly been some considerable alterations of level, excavation of valleys, deposits of great beds of gravel, and other superficial changes during this period, some kind of a scale of measurement of geological time has been obtained, by comparison with the very minute changes that have occurred during the historical period. This scale is admittedly a very imperfect one, but it is better than none at all; and it is by comparing these small changes with the far greater ones which have occurred during every successive step backward in geological history that these estimates of geological time have been arrived at. They are also supported by the palæontologists, to whom the vast panorama of successive forms of life is an ever-present reality. Directly they pass into the latest stage of the Tertiary period—the Pliocene of Sir Charles Lyell—all over the world new forms of life appear which are evidently the forerunners of many of our still existing species; and as they go a little further back, into the Miocene, there are indications of a warmer climate in Europe, and large numbers of mammals resembling those which now inhabit the tropics, but of quite distinct species and often of distinct genera and families. And here, though we have only reached to about the middle of the Tertiary period, the changes in the forms of life, in the climate, and in the land-surfaces are so great when compared with the very minute changes during the human epoch, as to require us to multiply the time elapsed many times over. Yet the whole of the Tertiary period, during which all the great groups of the higher animals were developed from a comparatively few generalised ancestral forms, is yet the shortest by far of the three great geological periods—the Mesozoic or Secondary, having been much longer, with still vaster changes both in the earth's crust and in the forms of life; while the Palæozoic or Primary, which carries us back to the earliest forms of life as represented by fossilised remains, is always estimated by geologists to be at least as long as the other two combined, and probably very much longer.

From these various considerations most geologists who have made any estimates of geological time from the period of the earliest fossiliferous rocks, have arrived at the conclusion that about 200 millions of years are required. But from the variety of the forms of life at this early period it is concluded that a very much greater duration is needed for the whole epoch of life. Speaking of the varied marine fauna of the Cambrian period, the late Professor Ramsay says:—'In this earliest known varied life we find no evidence of its having lived near the beginning of the zoological series. In a broad sense, compared with what must have gone before, both biologically and physically, all the phenomena connected with this old period seem, to my mind, to be of quite a recent description; and the climates of seas and lands were of the very same kind as those the world enjoys at the present day.' And Professor Huxley held very similar views when he declared: 'If the very small differences which are observable between the crocodiles of the older Secondary formations and those of the present day furnish any sort of an approximation towards an estimate of the average rate of change among reptiles, it is almost appalling to reflect how far back in Palæozoic times we must go before we can hope to arrive at that common stock from which the crocodiles, lizards, Ornithoscelida, and Plesiosauria, which had attained so great a development in the Triassic epoch, must have been derived.'

Now, in opposition to these demands of the geologists, in which they are almost unanimous, the most celebrated physicists, after full consideration of all possible sources of the heat of the sun, and knowing the rate at which it is now expending heat, declare, with complete conviction, that our sun cannot have existed as a heat-giving body for so long a period, and they would therefore reduce the time during which life can possibly have existed on the earth to about one-fourth of that demanded by geologists. In one of his latest articles, Lord Kelvin says:—'Now we have irrefragable dynamics proving that the whole life of our sun as a luminary is a very moderate number of million years, probably less than 50 million, possibly between 50 and 100' (Phil. Mag., vol. ii., Sixth Ser., p. 175, Aug. 1901). In my Island Life (chap. X.) I have myself given reasons for thinking that both the stratigraphical and biological changes may have gone on more quickly than has been supposed, and that geological time (meaning thereby the time during which the development of life upon the earth has been going on) may be reduced so as possibly to be brought within the maximum period allowed by physicists; but there will certainly be no time to spare, and any planets dependent on our sun whose period of habitability is either past or to come, cannot possibly have, or have had, sufficient time for the necessarily slow evolution of the higher life-forms. Again, all physicists hold that the sun is now cooling, and that its future life will be much less than its past. In a lecture at the Royal Institution (published in Nature Series, in 1889), Lord Kelvin says:—'It would, I think, be exceedingly rash to assume as probable anything more than twenty million years of the sun's light in the past history of the earth, or to reckon more than five or six million years of sunlight for time to come.'

These extracts serve to show that, unless either geologists or physicists are very far from any approach to accuracy in their estimates of past or future age of the sun, there is very great difficulty in bringing them into harmony or in accounting for the actual facts of the geological history of the earth and of the whole course of life-development upon it. We are, therefore, again brought to the conclusion that there has been, and is, no time to spare; that the whole of the available past life-period of the sun has been utilised for life-development on the earth, and that the future will be not much more than may be needed for the completion of the grand drama of human history, and the development of the full possibilities of the mental and moral nature of man.

We have here, then, a very powerful argument, from a different point of view than any previously considered, for the conclusion that man's place in the solar system is altogether unique, and that no other planet either has developed or can develop such a full and complete life-series as that which the earth has actually developed. Even if the conditions had been more favourable than they are seen to be on other planets, Mercury, Venus, and Mars could not possibly have preserved equability of conditions long enough for life-development, since for unknown ages they must have been passing slowly towards their present wholly unsuitable conditions; while Jupiter and the planets beyond him, whose epoch of life-development is supposed to be in the remote future when they shall have slowly cooled down to habitability, will then be still more faintly illuminated and scantily warmed by a rapidly cooling sun, and may thus become, at the best, globes of solid ice. This is the teaching of science—of the best science of the twentieth century. Yet we find even astronomers who, more than any other exponents of science, should give heed to the teachings of the sister sciences to which they owe so much, indulging in such rhapsodies as the following:—'In our solar system, this little earth has not obtained any special privileges from Nature, and it is strange to wish to confine life within the circle of terrestrial chemistry.' And again: 'Infinity encompasses us on all sides, life asserts itself, universal and eternal, our existence is but a fleeting moment, the vibration of an atom in a ray of the sun, and our planet is but an island floating in the celestial archipelago, to which no thought will ever place any bounds.'[20]

In place of such 'wild and whirling words,' I have endeavoured to state the sober conclusions of the best workers and thinkers as to the nature and origin of the world in which we live, and of the universe which on all sides surrounds us. I leave it to my readers to decide which is the more trustworthy guide.

CHAPTER XV

THE STARS—HAVE THEY PLANETARY SYSTEMS?

ARE THEY BENEFICIAL TO US?

Most of the writers on the Plurality of Worlds, from Fontenelle to Proctor, taking into consideration the enormous number of the stars and their apparent uselessness to our world, have assumed that many of them must have systems of planets circling round them, and that some of these planets, at all events, must possess inhabitants, some, perhaps, lower, but others no doubt higher than ourselves. One of our well-known modern astronomers, writing only ten years ago, adopts the same view. He says: 'The suns which we call stars were clearly not created for our benefit. They are of very little practical use to the earth's inhabitants. They give us very little light; an additional small satellite—one considerably smaller than the moon—would have been much more useful in this respect than the millions of stars revealed by the telescope. They must therefore have been formed for some other purpose.... We may therefore conclude, with a high degree of probability, that the stars—at least those with spectra of the solar type—form centres of planetary systems somewhat similar to our own.'[21] The author then discusses the conditions necessary for life analogous to that of our earth, as regards temperature, rotation, mass, atmosphere, water, etc., and he is the only writer I have met with who has considered these conditions; but he touches on them very briefly, and he arrives at the conclusion that, in the case of the stars of solar type, it is probable that one planet, situated at a proper distance, would be fitted to support life. He estimates roughly that there are about ten million stars of this type, that is, closely resembling our sun, and that if only one in ten of these has a planet at the proper distance and properly constituted in other respects, there will be one million worlds fitted for the support of animal life. He therefore concludes that there are probably many stars having life-bearing planets revolving round them.

There are, however, many considerations not taken account of by this writer which tend to reduce very considerably the above estimate. It is now known that immense numbers of the stars of smaller magnitudes are nearer to us than are the majority of the stars of the first and second magnitudes, so that it is probable that these, as well as a considerable proportion of the very faint telescopic stars, are really of small dimensions. We have evidence that many of the brightest stars are much larger than our sun, but there are probably ten times as many that are much smaller. We have seen that the whole of the past light and heat-giving duration of our sun has, according to the best authorities, been only just sufficient for the development of life upon the earth. But the duration of a sun's heat-giving power will depend mainly upon its mass, together with its constituent elements. Suns which are much smaller than ours are, therefore, from that cause alone, unsuited to give adequate light and heat for a sufficient time, and with sufficient uniformity, for life-development on planets, even if they possess any at the right distance, and with the extensive series of nicely adjusted conditions which I have shown to be necessary.

Again, we must, probably, rule out as unfitted for life-development the whole region of the Milky Way, on account of the excessive forces there in action, as shown by the immense size of many of the stars, their enormous heat-giving power, the crowding of stars and nebulous matter, the great number of star-clusters, and, especially, because it is the region of 'new stars,' which imply collisions of masses of matter sufficiently large to become visible from the immense distance we are from them, but yet excessively small as compared with suns the duration of whose light is to be measured by millions of years. Hence the Milky Way is the theatre of extreme activity and motion; it is comparatively crowded with matter undergoing continual change, and is therefore not sufficiently stable for long periods to be at all likely to possess habitable worlds.

We must, therefore, limit our possible planetary systems suitable for life-development, to stars situated inside the circle of the Milky Way and far removed from it—that is, to those composing the solar cluster. These have been variously estimated to consist of a few hundred or many thousand stars—at all events to a very small number as compared with the 'hundreds of millions' in the whole stellar universe. But even here we find that only a portion are probably suitable. Professor Newcomb arrives at the conclusion—as have some other astronomers—that the stars in general have a much smaller mass in proportion to the light they give than our sun has; and, after an elaborate discussion, he finally concludes that the brighter stars are, on the average, much less dense than our sun. In all probability, therefore, they cannot give light and heat for so long a period, and as this period in the case of our sun has only been just sufficient, the number of suns of the solar type and of a sufficient mass may be very limited. Yet further, even among stars having a similar physical constitution to our sun and of an equal or greater mass, only a portion of their period of luminosity would be suitable for the support of planetary life. While they are in process of formation by accretions of solid or gaseous masses, they would be subject to such fluctuations of temperature, and to such catastrophic outbursts when any larger mass than usual was drawn towards them, that the whole of this period—perhaps by far the longest portion of their existence—must be left out of the account of planet-producing suns. Yet all these are to us stars of various degrees of brilliancy. It is almost certain that it is only when the growth of a sun is nearly completed, and its heat has attained a maximum, that the epoch of life-development is likely to begin upon any planets it may possess at the most suitable distance, and upon which all the requisite conditions should be present.

It may be said that there are great numbers of stars beyond our solar cluster and yet within the circle of the Milky Way, as well as others towards the poles of the Milky Way, which I have not here referred to. But of these regions very little is known, because it is impossible to tell whether stars in these directions are situated in the outer portion of the solar cluster or in the regions beyond it. Some astronomers appear to think that these regions may be nearly empty of stars, and I have endeavoured to represent what seems to be the general view on this very difficult subject in the two diagrams of the stellar universe at pp. 300, 301. The regions beyond our cluster and above or below the plane of the Milky Way are those where the small irresolvable nebulæ abound, and these may indicate that sun-formation is not yet active in those regions. The two charts of Nebulæ and Clusters at the end of the volume illustrate, and perhaps tend to support this view.

Double and Multiple Star Systems

We have already seen, in our sixth chapter, how rapid and extraordinary has been the discovery of what are termed spectroscopic binaries—pairs of stars so close together as to appear like a single star in the most powerful telescopes. The systematic search for such stars has only been carried on for a few years, yet so many have been already found, and their numbers are increasing so rapidly, as to quite startle astronomers. One of the chief workers in this field, Professor Campbell of the Lick Observatory, has stated his opinion that, as accuracy of measurement increases, these discoveries will go on till—'the star that is not a spectroscopic binary will prove to be the rare exception,'—and other astronomers of eminence have expressed similar views. But these close revolving star-systems are generally admitted to be out of the category of life-producing suns. The tidal disturbances mutually produced must be enormous, and this must be inimical to the development of planets, unless they were very close to each sun, and thus in the most unfavourable position for life.

We thus see that the result of the most recent researches among the stars is entirely opposed to the old idea that the countless myriads of stars all had planets circulating round them, and that the ultimate purpose of their existence was, that they should be supporters of life, as our sun is the supporter of life upon the earth. So far is this from being the case, that vast numbers of stars have to be put aside as wholly unfitted for such a purpose; and when by successive eliminations of this nature we have reduced the numbers which may possibly be available to a few millions, or even to a few thousands, there comes the last startling discovery, that the entire host of stars is found to contain binary systems in such rapidly increasing numbers, as to lead some of the very first astronomers of the day to the conclusion that single stars may someday be found to be the rare exception! But this tremendous generalisation would, at one stroke, sweep away a large proportion of the stars which other successive disqualifications had spared, and thus leave our sun, which is certainly single, and perhaps two or three companion orbs, alone among the starry host as possible supporters of life on some one of the planets which circulate around them.

But we do not really know that any such suns exist. If they exist we do not know that they possess planets. If any do possess planets these may not be at the proper distance, or be of the proper mass, to render life possible. If these primary conditions should be fulfilled, and if there should possibly be not only one or two, but a dozen or more that so far fulfil the first few conditions which are essential, what probability is there that all the other conditions, all the other nice adaptations, all the delicate balance of opposing forces that we have found to prevail upon the earth, and whose combination here is due to exceptional conditions which exist in the case of no other known planet—should all be again combined in some of the possible planets of these possibly existing suns?

I submit that the probability is now all the other way. So long as we could assume that all the stars might be, in all essentials, like our sun, it seemed almost ludicrous to suppose that our sun alone should be in a position to support life. But when we find that enormous classes like the gaseous stars of small density, the solar stars while increasing in size and temperature, the stars which are much smaller than our sun, the nebulous stars, probably all the stars of the Milky Way, and lastly that enormous class of spectroscopic doubles—veritable Aaron's rods which threaten to swallow up all the rest—that all these are for various reasons unlikely to have attendant planets adapted to develop life, then the probabilities seem to be enormously against there being any considerable number of suns possessing attendant habitable earths. Just as the habitability of all the planets and larger satellites, once assumed as so extremely probable as to amount almost to a certainty, is now generally given up, so that in speculating on life in stellar systems Mr. Gore assumes that only one planet to each sun can be habitable; in like manner it may, and I believe will, turn out, that of all the myriad stars, the more we learn about them, the smaller and smaller will become the scanty residue which, with any probability, we can suppose to illuminate and vivify habitable earths. And when with this scanty probability we combine the still scantier probability that any such planet will possess simultaneously, and for a sufficiently long period, all the highly complex and delicately balanced conditions known to be essential for a full life-development, the conception that on this earth alone has such development been completed will not seem so wildly improbable a conjecture as it has hitherto been held to be.

Are the Stars Beneficial to Us?

When I suggested in my first publication on this subject that some emanations from the stars might be beneficial or injurious, and that a central position might be essential in order to render these emanations equable, one of my astronomical critics laughed the idea to scorn, and declared that 'we might wander into outer space without losing anything more serious than we lose when the night is cloudy and we cannot see the stars.'[22] How my critic knows that this is so he does not tell us. He states it positively, with no qualification, as if it were an established fact. It may be as well to inquire, therefore, if there is any evidence bearing upon the point at issue.

Astronomers are so fully occupied with the vast number and variety of the phenomena presented by the stellar universe and the various difficult problems arising therefrom, that many lesser but still interesting inquiries have necessarily received little attention. Such a minor problem is the determination of how much heat or other active radiation we receive from the stars; yet a few observations have been made with results that are of considerable interest.

In the years 1900 and 1901 Mr. E.F. Nichols of the Yerkes Observatory made a series of experiments with a radiometer of special construction, to determine the heat emitted by certain stars. The result arrived at was, that Vega gave about ¹/_200000000 of the heat of a candle at one metre distance, and Arcturus about 2.2 times as much.

In 1895 and 1896 Mr. G.M. Minchin made a series of experiments on the Electrical Measurement of Starlight, by means of a photo-electric cell of peculiar construction which is sensitive to the whole of the rays in the spectrum, and also to some of the ultra-red and ultra-violet rays. Combined with this was a very delicate electrometer. The telescope employed to concentrate the light was a reflector of two feet aperture. Mr. Minchin was assisted in the experiments by the late Professor G.F. Fitzgerald, F.R.S., of Trinity College, Dublin, which may be considered guarantee of the accuracy of the observations. The following are the chief results obtained:—

N.B.—The standard candle shone directly on the cell, whereas the star's light was concentrated by a 2-foot mirror.

The sensitive surface on which the light of the stars was concentrated was ¹/₂₀ inch in diameter. We must therefore diminish the amount of candle light in this table in the proportion of the square of the diameter of the mirror (in ¹/₂₀ of an inch) to one, equal to ¹/₂₃₀₄₀₀. If we make the necessary reduction in the case of Vega, and also equalise the distance at which the candle was placed, we find the following result:—

This enormous difference in the result is no doubt largely due to the fact that Mr. Nichols's apparatus measured heat alone, whereas Mr. Minchin's cell measured almost all the rays. And this is further shown by the fact that, whereas Mr. Nichols found Arcturus a red star, hotter than Vega a white one, Mr. Minchin, measuring also the light-giving and some of the chemical rays, found Vega considerably more energetic than Arcturus. These comparisons also suggest that other modes of measurement might give yet higher results, but it will no doubt be urged that such minute effects must necessarily be quite inoperative upon the organic world.

There are, however, some considerations which tend the other way. Mr. Minchin remarks on the unexpected fact that Betelgeuse produces more than double the electrical energy of Procyon, a much brighter star. This indicates that many of the stars of smaller visual magnitudes may give out a large amount of energy, and it is this energy, which we now know can take many strange and varied forms, that would be likely to influence organic life. And as to the quantity being too minute to have any effect, we know that the excessively minute amount of light from the very smallest telescopic stars produces such chemical changes on a photographic plate as to form distinct images, with comparatively small lenses or reflectors and with an exposure of two or three hours. And if it were not that the diffused light of the surrounding sky also acts upon the plate and blurs the faint images, much smaller stars could be photographed.

We know that not all the rays, but a portion only, are capable of producing these effects; we know also that there are many kinds of radiation from the stars, and probably some yet undiscovered comparable with the X rays and other new forms of radiation. We must also remember the endless variety and the extreme instability of the protoplasmic products in the living organism, many of which are perhaps as sensitive to special rays as is the photographic plate.

And we are not here limited to action for a few minutes or a few hours, but throughout the whole night and day, and continued whenever the sky is clear for months or years. Thus the cumulative effect of these very weak radiations may become important. It is probable that their action would be most influential on plants, and here we find all the conditions requisite for its accumulation and utilisation in the large amount of leaf-surface exposed to it. A large tree must present some hundreds of superficial feet of receptive surface, while even shrubs and herbs often have a leaf-area of greater superficial extent than the object-glasses of our largest telescopes. Some of the highly complex chemical processes that go on in plants may be helped by these radiations, and their action would be increased by the fact that, coming from every direction over the whole surface of the heavens, the rays from the stars would be able to reach and act upon every leaf of the densest masses of foliage. The large amount of growth that takes place at night may be in part due to this agency.

Of course all this is highly speculative; but I submit, in view of the fact that the light of the very faintest stars does produce distinct chemical changes, that even the very minute heat-effects are measureable, as well as the electro-motive forces caused by them; and further, that when we consider the millions, perhaps hundreds of millions of stars, all acting simultaneously on any organism which may be sensitive to them, the supposition that they do produce some effect, and possibly a very important effect, is not one to be summarily rejected as altogether absurd and not worth inquiring into.

It is not, however, these possible direct actions of the stars upon living organisms to which I attach much weight as regards our central position in the stellar universe. Further consideration of the subject has convinced me that the fundamental importance of that position is a physical one, as has already been suggested by Sir Norman Lockyer and some other astronomers. Briefly, the central position appears to be the only one where suns can be sufficiently stable and long-lived to be capable of maintaining the long process of life-development in any of the planets they may possess. This point will be further developed in the next (and concluding) chapter.

CHAPTER XVI

STABILITY OF THE STAR-SYSTEM: IMPORTANCE OF OUR

CENTRAL POSITION: SUMMARY AND CONCLUSION

One of the greatest difficulties with regard to the vast system of stars around us is the question of its permanence and stability, if not absolutely and indefinitely, yet for periods sufficiently long to allow for the many millions of years that have certainly been required for our terrestrial life-development. This period, in the case of the earth, as I have sufficiently shown, has been characterised throughout by extreme uniformity, while a continuance of that uniformity for a few millions of years in the future is almost equally certain.

But our mathematical astronomers can find no indications of such stability of the stellar universe as a whole, if subject to the law of gravitation alone. In reply to some questions on this point, my friend Professor George Darwin writes as follows:—'A symmetrical annular system of bodies might revolve in a circle with or without a central body. Such a system would be unstable. If the bodies are of unequal masses and not symmetrically disposed, the break-up of the system would probably be more rapid than in the ideal case of symmetry.'

This would imply that the great annular system of the Milky Way is unstable. But if so, its existence at all is a greater mystery than ever. Although in detail its structure is very irregular, as a whole it is wonderfully symmetrical; and it seems quite impossible that its generally circular ring-like form can be the result of the chance aggregation of matter from any pre-existing different form. Star-clusters are equally unstable, or, rather, nothing is known or can be predicated about their stability or instability, according to Professors Newcomb and Darwin.

Mr. E.T. Whittaker (Secretary to the Royal Astronomical Society), to whom Professor G. Darwin sent my questions, writes:—'I doubt whether the principal phenomena of the stellar universe are consequences of the law of gravitation at all. I have been working myself at spiral nebulæ, and have got a first approximation to an explanation—but it is electro-dynamical and not gravitational. In fact, it may be questioned whether, for bodies of such tremendous extent as the Milky Way or nebulæ, the effect which we call gravitation is given by Newton's law; just as the ordinary formulæ of electrostatic attraction break down when we consider charges moving with very great velocities.'

Accepting these statements and opinions of two mathematicians who have given special attention to similar problems, we need not limit ourselves to the laws of gravitation as having determined the present form of the stellar universe; and this is the more important because we may thus escape from a conclusion which many astronomers seem to think inevitable, viz. that the observed proper motions of the stars cannot be explained by the gravitative forces of the system itself. In chapter VIII. of this work I have quoted Professor Newcomb's calculation as to the effect of gravitation in a universe of 100 million stars, each five times the mass of our sun, and spread over a sphere which it would take light 30,000 years to cross; then, a body falling from its outer limits to the centre could at the utmost acquire a velocity of twenty-five miles a second; and therefore, any body in any part of such a universe having a greater velocity would pass away into infinite space. Now, as several stars have, it is believed, much more than this velocity, it follows not only that they will inevitably escape from our universe, but that they do not belong to it, as their great velocity must have been acquired elsewhere. This seems to have been the idea of the astronomer who stated that, even at the very moderate speed of our sun, we should in five million years be deep in the actual stream of the Milky Way. To this I have already sufficiently replied; but I now wish to bring before my readers an excellent illustration of the importance of the late Professor Huxley's remark, that the results you got out of the 'mathematical mill' depend entirely on what you put into it.

In the Philosophical Magazine (January 1902) is a remarkable article by Lord Kelvin, in which he discusses the very same problem as that which Professor Newcomb had discussed at a much earlier date, but, starting from different assumptions, equally based on ascertained facts and probabilities deduced from them, brings out a very different result.

Lord Kelvin postulates a sphere of such a radius that a star at its confines would have a parallax of one-thousandth part of a second (0".001), equivalent to 3215 light-years. Uniformly distributed through this sphere there is matter equal in mass to 1000 million suns like ours. If this matter becomes subject to gravitation, it all begins to move at first with almost infinite slowness, especially near its centre; but nevertheless, in twenty-five million years many of these suns would have acquired velocities of from twelve to twenty miles a second, while some would have less and some probably more than seventy miles a second. Now such velocities as these agree generally with the measured velocities of the stars, hence Lord Kelvin thinks there may be as much matter as 1000 million suns within the above-named distance. He then states that if we suppose there to be 10,000 million suns within the same sphere, velocities would be produced very much greater than the known star-velocities; hence it is probable that there is very much less matter than 10,000 million times the sun's mass. He also states that if the matter were not uniformly distributed within the sphere, then, whatever was the irregularity, the acquired motions would be greater; again indicating that the 1000 million suns would be ample to produce the observed effects of stellar motion. He then calculates the average distance apart of each of the 1000 million stars, which he finds to be about 300 millions of millions of miles. Now the nearest star to our sun is about twenty-six million million of miles distant, and, as the evidence shows, is situated in the denser part of the solar cluster. This gives ample allowance for the comparative emptiness of the space between our cluster and the Milky Way, as well as of the whole region towards the poles of the Milky Way (as shown by the diagrams in chapter IV.), while the comparative density of extensive portions of the Galaxy itself may serve to make up the average.

Now, previous writers have come to a different conclusion from the same general line of argument, because they have started with different assumptions. Professor Newcomb, whose statement made some years back is usually followed, assumed 100 million stars each five times as large as our sun, equal to 500 million suns in all, and he distributed them equally throughout a sphere 30,000 light-years in diameter. Thus he has half the amount of matter assumed by Lord Kelvin, but nearly five times the extent, the result being that gravity could only produce a maximum speed of twenty-five miles a second; whereas on Lord Kelvin's assumption a maximum speed of seventy miles a second would be produced, or even more. By this latter calculation we find no insuperable difficulty in the speed of any of the stars being beyond the power of gravitation to produce, because the rates here given are the direct results of gravitation acting on bodies almost uniformly distributed through space. Irregular distribution, such as we see everywhere in the universe, might lead to both greater and less velocities; and if we further take account of collisions and near approaches of large masses resulting in explosive disruptions, we might have almost any amount of motion as the result, but as this motion would be produced by gravitation within the system, it could equally well be controlled by gravitation.

In order that my readers may better understand the calculations of Lord Kelvin, and also the general conclusions of astronomers as to the form and dimensions of the stellar universe, I have drawn two diagrams, one showing a plan on the central plane of the Milky Way, the other a section through its poles. Both are on the same scale, and they show the total diameter across the Milky Way as being 3600 light-years, or about half that postulated by Lord Kelvin for his hypothetical universe. I do this because the dimensions given by him are those which are sufficient to lead to motions near the centre such as the stars now possess in a minimum period of twenty-five million years after the initial arrangement he supposes, at which later epoch which we are now supposed to have reached, the whole system would of course be greatly reduced in extent by aggregations towards and near the centre. These dimensions also seem to accord sufficiently with the actual distances of stars as yet measured. The smallest parallax which has been determined with any certainty, according to Professor Newcomb's list, is that of Gamma Cassiopeiæ, which is one-hundredth of a second (0".01), while Lord Kelvin gives none smaller than 0".02, and these will all be included within the solar cluster as I have shown it.

It must be clearly understood that these two illustrations are merely diagrams to show the main features of the stellar universe according to the best information available, with the proportionate dimensions of these features, so far as the facts of the distribution of the stars and the views of those astronomers who have paid most attention to the subject can be harmonised. Of course it is not suggested that the whole arrangement is so regular as here shown, but an attempt has been made by means of the dotted shading to represent the comparative densities of the different portions of space around us, and a few remarks on this point may be needed.

The solar cluster is shown very dense at the central portion, occupying one-tenth of its diameter, and it is near the outside of this dense centre that our sun is supposed to be situated. Beyond this there seems to be almost a vacuity, beyond which again is the outer portion of the cluster consisting of comparatively thinly scattered stars, thus forming a kind of ring-cluster, resembling in shape the beautiful ring-nebula in Lyra, as has been suggested by several astronomers. There is some direct evidence for this ring-form. Professor Newcomb in his recent book on The Stars gives a list of all stars of which the parallax is fairly well known. These are sixty-nine in number; and on arranging them in the order of the amount of their parallax, I find that no less than thirty-five of them have parallaxes between 0".1 and 0".4 of a second, thus showing that they constitute part of the dense central mass; while three others, from 0".4 to 0".75, indicate those which are our closest companions at the present time, but still at an enormous distance. Those which have parallaxes of less than the tenth and down to one-hundredth of a second are only thirty-one in all; but as they are spread over a sphere ten times the diameter, and therefore a thousand times the cubic content of the sphere containing those above one-tenth of a second, they ought to be immensely more numerous even if very much more thinly scattered. The interesting point, however, is, that till we get down to a parallax of 0".06, there are only three stars as yet measured, whereas those between 0".02 and 0".06, an equal range of parallax, are twenty-six in number, and as these are scattered in all directions they indicate an almost vacant space followed by a moderately dense outer ring.

In the enormous space between our cluster and the Milky Way, and also above and below its plane to the poles of the Galaxy, stars appear to be very thinly scattered, perhaps more densely in the plane of the Milky Way than above and below it where the irresolvable nebulæ are so numerous; and there may not improbably be an almost vacant space beyond our cluster for a considerable distance, as has been supposed, but this cannot be known till some means are discovered of measuring parallaxes of from one-hundredth to one five-hundredth of a second.

These diagrams also serve to indicate another point of considerable importance to the view here advocated. By placing the solar system towards the outer margin of the dense central portion of the solar cluster (which may very possibly include a large proportion of dark stars and thus be much more dense towards the centre than it appears to us), it may very well be supposed to revolve, with the other stars composing it, around the centre of gravity of the cluster, as the force of gravity towards that centre might be perhaps twenty or a hundred times greater than towards the very much less dense and more remote outer portions of the cluster. The sun, as indicated on the diagrams, is about thirty light-years from that centre, corresponding to a parallax of a little more than one-tenth of a second, and an actual distance of 190 millions of millions of miles, equal to about 70,000 times the distance of the sun from Neptune. Yet we see that this position is so little removed from the exact centre of the whole stellar universe, that if any beneficial influences are due to that central position in regard to the Galaxy, it will receive them perhaps to as full an extent as if situated at the actual centre. But if it is situated as here shown, there is no further difficulty as to its proper motion carrying it from one side to the other of the Milky Way in less time than has been required for the development of life upon the earth. And if the solar cluster is really sub-globular, and sufficiently condensed to serve as a centre of gravity for the whole of the stars of the cluster to revolve around, all the component stars which are not situated in the plane of its equator (and that of the Milky Way) must revolve obliquely at various angles up to an angle of 90°. These numerous diverging motions, together with the motions of the nearer stars outside the cluster, some of which may revolve round other centres of gravity made up largely of dark bodies, would perhaps sufficiently account for the apparent random motions of so many of the stars.

Uniform Heat-Supply due to Central Position

We now come to a point of the greatest interest as regards the problem we are investigating. We have seen how great is the difference in the estimates of geologists and those of physicists as to the time that has elapsed during the whole development of life. But the position we have now found for the sun, in the outer portion of the central star-cluster, may afford a clue to this problem. What we require is, some mode of keeping up the sun's heat during the enormous geological periods in which we have evidence of a wonderful uniformity in the earth's temperature, and therefore in the sun's heat-emission. The great central ring-cluster with its condensed central mass, which presumably has been forming for a much longer period than our sun has been giving heat to the earth, must during all this time have been exerting a powerful attraction on the diffused matter in the spaces around it, now apparently almost void as compared with what they may have been. Some scanty remnants of that matter we see in the numerous meteoric swarms which have been drawn into our system. A position towards the outside of this central aggregation of suns would evidently be very favourable for the growth by accretion of any considerable mass. The enormous distance apart of the outer components (the outer ring) of the cluster would allow a large amount of the inflowing meteoritic matter to escape them, and the larger suns situated near the surface of the inner dense cluster would draw to themselves the greater part of this matter.[23] The various planets of our system were no doubt built up from a portion of the matter that flowed in near the plane of the ecliptic, but much of that which came from all other directions would be drawn towards the sun itself or to its neighbouring suns. Some of this would fall directly into it; other masses coming from different directions and colliding with each other would have their motion checked, and thus again fall into the sun; and so long as the matter falling in were not in too large masses, the slow additions to the sun's bulk and increase of its heat would be sufficiently gradual to be in no way prejudicial to a planet at the earth's distance.

The main point I wish to suggest here is, that by far the greater portion of the matter of the whole stellar universe has, either through gravitation or in combination with electrical forces, as suggested by Mr. Whittaker, become drawn together into the vast ring-formed system of the Milky Way, which is, presumably, slowly revolving, and has thus been checked in its original inflow toward the centre of mass of the stellar universe. It has also probably drawn towards itself the adjacent portions of the scattered material in the spaces around it in all directions.

Had the vast mass of matter postulated by Lord Kelvin acquired no motion of revolution, but have fallen continuously towards the centre of mass, the motions developed when the more distant bodies approached that centre would have been extremely rapid; while, as they must have fallen in from every direction, they would have become more and more densely aggregated, and collisions of the most catastrophic nature would frequently have occurred, and this would have rendered the central portion of the universe the least stable and the least fitted to develop life.

But, under the conditions that actually prevail, the very reverse is the case. The quantity of matter remaining between our cluster and the Milky Way being comparatively small, the aggregation into suns has gone on more regularly and more slowly. The motions acquired by our sun and its neighbours have been rendered moderate by two causes: (1) their nearness to the centre of the very slowly aggregating cluster where the motion due to gravitation is least in amount; and (2) the slight differential attraction away from the centre by the Milky Way on the side nearest to us. Again, this protective action of the Milky Way has been repeated, on a smaller scale, by the formation of the outer ring of the solar cluster, which has thus preserved the inner central cluster itself from a too abundant direct inflow of large masses of matter.

But although the matter composing the outer portion of the original universe has been to a large extent aggregated into the vast system of the Milky Way, it seems probable, perhaps even certain, that some portion would escape its attractive forces and would pass through its numerous open spaces—indicated by the dark rifts, channels, and patches, as already described—and thus flow on unchecked towards the centre of mass of the whole system. The quantity of matter thus reaching the central cluster from the enormously remote spaces beyond the Milky Way might be very small in comparison with what was retained to build up that wonderful star-system; but it might yet be so large in total amount as to play an important part in the formation of the central group of suns. It would probably flow inwards almost continuously, and when it ultimately reached the solar cluster, it would have attained a very high velocity. If, therefore, it were widely diffused, and consisted of masses of small or moderate size as compared with planets or stars, it would furnish the energy requisite for bringing these slowly aggregating stars to the required intensity of heat for forming luminous suns.

Here, then, I think, we have found an adequate explanation of the very long-continued light and heat-emitting capacity of our sun, and probably of many others in about the same position in the solar cluster. These would at first gradually aggregate into considerable masses from the slowly moving diffused matter of the central portions of the original universe; but at a later period they would be reinforced by a constant and steady inrush of matter from its very outer regions, and therefore possessing such high velocities as to materially aid in producing and maintaining the requisite temperature of a sun such as ours, during the long periods demanded for continuous life-development. The enormous extension and mass of the original universe of diffused matter (as postulated by Lord Kelvin) is thus seen to be of the greatest importance as regards this ultimate product of evolution, because, without it, the comparatively slow-moving and cool central regions might not have been able to produce and maintain the requisite energy in the form of heat; while the aggregation of by far the larger portion of its matter in the great revolving ring of the galaxy was equally important, in order to prevent the too great and too rapid inflow of matter to these favoured regions.

It appears, then, that if we admit as probable some such process of development as I have here indicated, we can dimly see the bearing of all the great features of the stellar universe upon the successful development of life. These are, its vast dimensions; the form it has acquired in the mighty ring of the Milky Way; and our position near to, but not exactly in, its centre. We know that the star-system has acquired these forms, presumably from some simple and more diffused condition. We know that we are situated near the centre of this vast system. We know that our sun has emitted light and heat, almost uniformly, for periods incompatible with rapid aggregation and the equally rapid cooling which physicists consider inevitable. I have here suggested a mode of development which would lead to a very slow but continuous growth of the more central suns; to an excessively long period of nearly stationary heat-giving power; and lastly, an equally long period of very gradual cooling—a period the commencement of which our sun may have just entered upon.

Descending now to terrestrial physics, I have shown that, owing to the highly complex nature of the adjustments required to render a world habitable and to retain its habitability during the æons of time requisite for life-development, it is in the highest degree improbable that the required conditions and adaptations should have occurred in any other planets of any other suns, which might occupy an equally favourable position with our own, and which were of the requisite size and heat-giving power.

Lastly, I submit that the whole of the evidence I have here brought together leads to the conclusion that our earth is almost certainly the only inhabited planet in our solar system; and, further, that there is no inconceivability—no improbability even—in the conception that, in order to produce a world that should be precisely adapted in every detail for the orderly development of organic life culminating in man, such a vast and complex universe as that which we know exists around us, may have been absolutely required.

Summary of Argument

As the last ten chapters of this volume embody a connected argument leading to the conclusion above stated, it may be useful to my readers to summarise rather fully the successive steps of this argument, the facts on which it rests, and the various subsidiary conclusions arrived at.

(1) One of the most important results of modern astronomy is to have established the unity of the vast stellar universe which we see around us. This rests upon a great mass of observations, which demonstrate the wonderful complexity in detail of the arrangement and distribution of stars and nebulæ, combined with a no less remarkable general symmetry, indicating throughout a single inter-dependent system, not a number of totally distinct systems so far apart as to have no physical relations with each other, as was once supposed.

(2) This view is supported by numerous converging lines of evidence, all tending to show that the stars are not infinite in number, as was once generally believed, and which view is even now advocated by some astronomers. The very remarkable calculations of Lord Kelvin, referred to in the early part of this chapter, give a further support to this view, since they show that if the stars extended much beyond those we see or can obtain direct knowledge of, and with no very great change in their average distance apart, then the force of gravitation towards the centre would have produced on the average more rapid motions than the stars generally possess.

(3) An overwhelming consensus of opinion among the best astronomers establishes the fact of our nearly central position in the stellar universe. They all agree that the Milky Way is nearly circular in form. They all agree that our sun is situated almost exactly in its medial plane. They all agree that our sun, although not situated at the exact centre of the galactic circle, is yet not very far from it, because there are no unmistakable signs of our being nearer to it at any one point and farther away from the opposite point. Thus the nearly central position of our sun in the great star-system is almost universally admitted.

On the question of the solar-cluster there is more difference of opinion; though here, again, all are agreed that there is such a cluster. Its size, form, density, and exact position are somewhat uncertain, but I have, as far as possible, been guided by the best available evidence. If we adopt Lord Kelvin's general idea of the gradual condensation of an enormous diffused mass of matter towards its common centre of gravity, that centre would be approximately the centre of this cluster. Also, as gravitational force at and near this centre would be comparatively small, the motions produced there would be slow, and collisions, being due only to differential motions, when they did occur would be very gentle. We might therefore expect many dark aggregations of matter here, which may explain why we do not find any special crowding of visible stars in the direction of this centre; while, as no star has a sensible disc, the dark stars if at great distances would hardly ever be seen to occult the bright ones. Thus, it seems to me, the controlling force may be explained which has retained our sun in approximately the same orbit around the centre of gravity of this central cluster during the whole period of its existence as a sun and our existence as a planet; and has thus saved us from the possibility—perhaps even the certainty—of disastrous collisions or disruptive approaches to which suns, in or near the Milky Way, and to a less extent elsewhere, are or have been exposed. It seems quite probable that in that region of more rapid and less controlled motions and more crowded masses of matter, no star can remain in a nearly stable condition as regards temperature for sufficiently long periods to allow of a complete system of life-development on any planet it may possess.

(4) The various proofs are next stated that assure us of the almost complete uniformity of matter, and of material physical and chemical laws, throughout our universe. This I believe no one seriously disputes; and it is a point of the greatest importance when we come to consider the conditions required for the development and maintenance of life, since it assures us that very similar, if not identical, conditions must prevail wherever organic life is or can be developed.

(5) This leads us on to the consideration of the essential characteristics of the living organism, consisting as it does of some of the most abundant and most widely distributed of these material elements, and being always subject to the general laws of matter. The best authorities in physiology are quoted, as to the extreme complexity of the chemical compounds which constitute the physical basis for the manifestation of life; as to their great instability; their wonderful mobility combined with permanence of form and structure; and the altogether marvellous powers they possess of bringing about unique chemical transformations and of building up the most complicated structures from simple elements.

I have endeavoured to put the broad phenomena of vegetable and animal life in a way that will enable my readers to form some faint conception of the intricacy, the delicacy, and the mystery of the myriad living forms they see everywhere around them. Such a conception will enable them to realise how supremely grand is organic life, and to appreciate better, perhaps, the absolute necessity for the numerous, complex and delicate adaptations of inorganic nature, without which it is impossible for life either to exist now, or to have been developed during the immeasurable past.

(6) The general conditions which are absolutely essential for life thus manifested on our planet are then discussed, such as, solar light and heat; water universally distributed on the planet's surface and in the atmosphere; an atmosphere of sufficient density, and composed of the several gases from which alone protoplasm can be formed; some alternations of light and darkness, and a few others.

(7) Having treated these conditions broadly, and explained why they are important and even indispensable for life, we next proceed to show how they are fulfilled upon the earth, and how numerous, how complex, and often how exact are the adjustments needed to bring them about, and maintain them almost unchanged throughout the vast æons of time occupied in the development of life. Two chapters are devoted to this subject; and it is believed that they contain facts that will be new to many of my readers. The combinations of causes which lead to this result are so varied, and in several cases dependent on such exceptional peculiarities of physical constitution, that it seems in the highest degree improbable that they can all be found again combined either in the solar system or even in the stellar universe. It will be well here just to enumerate these conditions, which are all essential within more or less narrow limits:—

Distance of planet from the sun.

Mass of planet.

Obliquity of its ecliptic.

Amount of water as compared with land.

Surface distribution of land and water.

Permanence of this distribution, dependent probably on the unique origin of our moon.

An atmosphere of sufficient density, and of suitable component gases.

An adequate amount of dust in the atmosphere.

Atmospheric electricity.

Many of these act and react on each other, and lead to results of great complexity.

(8) Passing on to other planets of the solar system, it is shown that none of them combine all the complex conditions which are found to work harmoniously together on the earth; while in most cases there is some one defect which alone removes them from the category of possible life-producing and life-supporting planets. Among these are the small size and mass of Mars, being such that it cannot retain aqueous vapour; and the fact that Venus rotates on its axis in the same time as it takes to revolve round the sun. Neither of these facts was known when Proctor wrote upon the question of the habitability of the planets. All the other planets are now given up—and were given up by Proctor himself—as possible life-bearers in their present stage; but he and others have held that, if not suitable now, some of them may have been the scene of life-development in the past, while others will be so in the future.

In order to show the futility of this supposition, the problem of the duration of the sun as a stable heat-giver is discussed; and it is shown that it is only by reducing the periods claimed by geologists and biologists for life-development upon the earth, and by extending the time allowed by physicists to its utmost limits, that the two claims can be harmonised. It follows that the whole period of the sun's duration as a light and heat-giver has been required for the development of life upon the earth; and that it is only upon planets whose phases of development synchronise with that of the earth that the evolution of life is possible. For those whose material evolution has gone on quicker or slower, there has not been, or will not be, time enough for the development of life.

(9) The problem of the stars as possibly having life-supporting planets is next dealt with, and reasons are given why in only a minute portion of the whole is this possible. Even in that minute portion, reduced probably to a few of the component suns of the solar-cluster, a large proportion seems likely to be ruled out by being close binary systems, and another large portion by being in process of aggregation. In those remaining, whether they may be reckoned by tens or by hundreds we cannot say, the chances against the same complex combination of conditions as those which we find on the earth occurring on any planet of any other sun are enormously great.

(10) I then refer, briefly, to some recent measurements of star-radiation, and suggest that they may thus possibly have important effects on the development of vegetable and animal life; and, finally, I discuss the problem of the stability of the stellar universe and the special advantage we derive from our central position, suggested by some of the latest researches of our great mathematician and physicist—Lord Kelvin.

Conclusions

Having thus brought together the whole of the available evidence bearing upon the questions treated in this volume, I claim that certain definite conclusions have been reached and proved, and that certain other conclusions have enormous probabilities in their favour.

The conclusions reached by modern astronomers are: (1) That the stellar universe forms one connected whole; and, though of enormous extent, is yet finite, and its extent determinable.

(2) That the solar system is situated in the plane of the Milky Way, and not far removed from the centre of that plane. The earth is therefore nearly in the centre of the stellar universe.

(3) That this universe consists throughout of the same kinds of matter, and is subjected to the same physical and chemical laws.

The conclusions which I claim to have shown to have enormous probabilities in their favour are—

(4) That no other planet in the solar system than our earth is inhabited or habitable.

(5) That the probabilities are almost as great against any other sun possessing inhabited planets.

(6) That the nearly central position of our sun is probably a permanent one, and has been specially favourable, perhaps absolutely essential, to life-development on the earth.

These latter conclusions depend upon the combination of a large number of special conditions, each of which must be in definite relation to many of the others, and must all have persisted simultaneously during enormous periods of time. The weight to be given to this kind of reasoning depends upon a full and fair consideration of the whole evidence as I have endeavoured to present it in the last seven chapters of this book. To this evidence I appeal.

This completes my work as a connected argument, founded wholly on the facts and principles accumulated by modern science; and it leads, if my facts are substantially correct and my reasoning sound, to one great and definite conclusion—that man, the culmination of conscious organic life, has been developed here only in the whole vast material universe we see around us. I claim that this is the logical outcome of the evidence, if we consider and weigh this evidence without any prepossessions whatever. I maintain that it is a question as to which we have no right to form a priori opinions not founded upon evidence. And evidence opposed to this conclusion, or even as to its improbability, we have absolutely none whatever.

But, if we admit the conclusion, nothing that need alarm either the scientific or the religious mind necessarily follows, because it can be explained or accounted for in either of two distinct ways. One considerable body, including probably the majority of men of science, will admit that the evidence does apparently lead to this conclusion, but will explain it as due to a fortunate coincidence. There might have been a hundred or a thousand life-bearing planets, had the course of evolution of the universe been a little different, or there might have been none at all. They would probably add, that, as life and man have been produced, that shows that their production was possible; and therefore, if not now then at some other time, if not here then in some other planet of some other sun, we should be sure to have come into existence; or if not precisely the same as we are, then something a little better or a little worse.