Other worlds and their inhabitants are remarkably popular subjects of speculation at the present time. Every day we hear people asking one another if it is true that we shall soon be able to communicate with some of the far-off globes, such as Mars, that circle in company with our earth about the sun. One of the masters of practical electrical science in our time has suggested that the principle of wireless telegraphy may be extended to the transmission of messages across space from planet to planet. The existence of intelligent inhabitants in some of the other planets has become, with many, a matter of conviction, and for everybody it presents a question of fascinating interest, which has deeply stirred the popular imagination.

The importance of this subject as an intellectual phenomenon of the opening century is clearly indicated by the extent to which it has entered into recent literature. Poets feel its inspiration, and novelists and romancers freely select other planets as the scenes of their stories. One tells us of a visit paid by men to the moon, and of the wonderful things seen, and adventures had, there. Lucian, it is true, did the same thing eighteen hundred years ago, but he had not the aid of hints from modern science to guide his speculations and lend verisimilitude to his narrative.

Another startles us from our sense of planetary security with a realistic account of the invasion of the earth by the terrible sons of warlike Mars, seeking to extend their empire by the conquest of foreign globes.

Sometimes it is a trip from world to world, a kind of celestial pleasure yachting, with depictions of creatures more wonderful than—

that is presented to our imagination; and sometimes we are informed of the visions beheld by the temporarily disembodied spirits of trance mediums, or other modern thaumaturgists, flitting about among the planets.

Then, to vary the theme, we find charming inhabitants of other worlds represented as coming down to the earth and sojourning for a time on our dull planet, to the delight of susceptible successors of father Adam, who become, henceforth, ready to follow their captivating visitors to the ends of the universe.

In short, writers of fiction have already established interplanetary communication to their entire satisfaction, thus vastly and indefinitely enlarging the bounds of romance, and making us so familiar with the peculiarities of our remarkable brothers and sisters of Mars, Venus, and the moon, that we can not help feeling, notwithstanding the many divergences in the descriptions, that we should certainly recognize them on sight wherever we might meet them.

But the subject is by no means abandoned to the tellers of tales and the dreamers of dreams. Men of science, also, eagerly enter into the discussion of the possibilities of other worlds, and become warm over it.

Around Mars, in particular, a lively war of opinions rages. Not all astronomers have joined in the dispute—some have not imagination enough, and some are waiting for more light before choosing sides—but those who have entered the arena are divided between two opposed camps. One side holds that Mars is not only a world capable of having inhabitants, but that it actually has them, and that they have given visual proof of their existence and their intelligence through the changes they have produced upon its surface. The other side maintains that Mars is neither inhabited nor habitable, and that what are taken for vast public works and engineering marvels wrought by its industrious inhabitants, are nothing but illusions of the telescope, or delusions of the observer's mind. Both adduce numerous observations, telescopic and spectroscopic, and many arguments, scientific and theoretic, to support their respective contentions, but neither side has yet been able to convince or silence the other, although both have made themselves and their views intensely interesting to the world at large, which would very much like to know what the truth really is.

And not only Mars, but Venus—the beauteous twin sister of the earth, who, when she glows in the evening sky, makes everybody a lover of the stars—and even Mercury, the Moor among the planets, wearing "the shadowed livery of the burnished sun," to whom he is "a neighbor and near bred," and Jupiter, Saturn, and the moon itself—all these have their advocates, who refuse to believe that they are lifeless globes, mere reflectors of useless sunshine.

The case of the moon is, in this respect, especially interesting, on account of the change that has occurred in the opinions held concerning its physical condition. For a very long time our satellite was confidently, and almost universally, regarded as an airless, waterless, lifeless desert, a completely "dead world," a bare, desiccated skull of rock, circling about the living earth.

But within a few years there has been a reaction from this extreme view of the lifelessness of the moon. Observers tell us of clouds suddenly appearing and then melting to invisibility over volcanic craters; of evidences of an atmosphere, rare as compared with ours, yet manifest in its effects; of variations of color witnessed in certain places as the sunlight drifts over them at changing angles of incidence; of what seem to be immense fields of vegetation covering level ground, and of appearances indicating the existence of clouds of ice crystals and deposits of snow among the mountainous lunar landscapes. Thus, in a manner, the moon is rehabilitated, and we are invited to regard its silvery beams not as the reflections of the surface of a desert, but as sent back to our eyes from the face of a world that yet has some slight remnants of life to brighten it.

The suggestion that there is an atmosphere lying close upon the shell of the lunar globe, filling the deep cavities that pit its face and penetrating to an unknown depth in its interior, recalls a speculation of the ingenious and entertaining Fontenelle, in the seventeenth century—recently revived and enlarged upon by the author of one of our modern romances of adventure in the moon—to the effect that the lunar inhabitants dwell beneath the surface of their globe instead of on the top of it.

Now, because of this widespread and continually increasing interest in the subject of other worlds, and on account of the many curious revelations that we owe to modern telescopes and other improved means of investigation, it is certainly to be desired that the most important and interesting discoveries that have lately been made concerning the various globes which together with the earth constitute the sun's family, should be assembled in a convenient and popular form—and that is the object of this book. Fact is admittedly often stranger and more wonderful than fiction, and there are no facts that appeal more powerfully to the imagination than do those of astronomy. Technical books on astronomy usually either ignore the subject of the habitability of the planets, or dismiss it with scarcely any recognition of the overpowering human interest that it possesses. Hence, a book written specially from the point of view of that subject would appear calculated to meet a popular want; and this the more, because, since Mr. Proctor wrote his Other Worlds than Ours and M. Flammarion his PluralitÉ des Mondes HabitÉs, many most important and significant discoveries have been made that, in several notable instances, have completely altered the aspect in which the planets present themselves for our judgment as to their conditions of habitability.

No doubt the natural tendency of the mind is to regard all the planets as habitable worlds, for there seems to be deeply implanted in human nature a consciousness of the universality of life, giving rise to a conviction that one world, even in the material sense, is not enough for it, but that every planet must belong to its kingdom. We are apt to say to ourselves: "The earth is one of a number of planets, all similarly circumstanced; the earth is inhabited, why should not the others also be inhabited?"

What has been learned of the unity in chemical constitution and mechanical operation prevailing throughout the solar system, together with the continually accumulating evidence of the common origin of its various members, and the identity of the evolutionary processes that have brought them into being, all tends to strengthen the a priori hypothesis that life is a phenomenon general to the entire system, and only absent where its essential and fundamental conditions, for special and local, and perhaps temporary, reasons, do not exist.

If we look for life in the sun, for instance, while accepting the prevalent conception of the sun as a center of intense thermal action, we must abandon all our ideas of the physical organization of life formed upon what we know of it from experimental evidence. We can not imagine any form of life that has ever been presented to our senses as existing in the sun.

But this is not generally true of the planets. Life, in our sense of it, is a planetary, not a solar, phenomenon, and while we may find reasons for believing that on some of the planets the conditions are such that creatures organized like ourselves could not survive, yet we can not positively say that every form of living organism must necessarily be excluded from a world whose environment would be unsuited for us and our contemporaries in terrestrial life.

Although our sole knowledge of animated nature is confined to what we learn by experience on the earth, yet it is a most entertaining, and by no means unedifying, occupation, to seek to apply to the exceedingly diversified conditions prevailing in the other planets, as astronomical observations reveal them to us, the principles, types, and limitations that govern the living creatures of our world, and to judge, as best we can, how far those types and limits may be modified or extended so that those other planets may reasonably be included among the probable abodes of life.

In order to form such judgments each planet must be examined by itself, but first it is desirable to glance at the planetary system as a whole. To do this we may throw off, in imagination, the dominance of the sun, and suppose ourselves to be in the midst of open space, far removed both from the sun and the other stars. In this situation it is only by chance, or through foreknowledge, that we can distinguish our sun at all, for it is lost among the stars; and when we discover it we find that it is only one of the smaller and less conspicuous members of the sparkling host.

We rapidly approach, and when we have arrived within a distance comparable with that of its planets, we see that the sun has increased in apparent magnitude, until now it enormously outshines all the other stars, and its rays begin to produce the effect of daylight upon the orbs that they reach. But we are in no danger of mistaking its apparent superiority to its fellow stars for a real one, because we clearly perceive that our nearness alone makes it seem so great and overpowering.

And now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. It is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. Although they vary in relative size, yet we observe a striking similarity among them. They are all globular bodies, they all turn upon their axes, they all travel about the sun in the same direction, and their paths all lie very nearly in one plane. Some of them have one or more moons, or satellites, circling about them in imitation of their own revolution about the sun. Their family relationship to one another and to the sun is so evident that it colors our judgment about them as individuals; and when we happen to find, upon closer approach, that one of them, the earth, is covered with vegetation and water and filled with thousands of species of animated creatures, we are disposed to believe, without further examination, that they are all alike in this respect, just as they are all alike in receiving light and heat from the sun.

This preliminary judgment, arising from the evident unity of the planetary system, can only be varied by an examination of its members in detail.

One striking fact that commands our attention as soon as we have entered the narrow precincts of the solar system is the isolation of the sun and its attendants in space. The solar system occupies a disk-shaped, or flat circular, expanse, about 5,580,000,000 miles across and relatively very thin, the sun being in the center. From the sun to the nearest star, or other sun, the distance is approximately five thousand times the entire diameter of the solar system. But the vast majority of the stars are probably a hundred times yet more remote. In other words, if the Solar system be represented by a circular flower-bed ten feet across, the nearest star must be placed at a distance of nine and a half miles, and the great multitude of the stars at a distance of nine hundred miles!

Or, to put it in another way, let us suppose the sun and his planets to be represented by a fleet of ships at sea, all included within a space about half a mile across; then, in order that there might be no shore relatively nearer than the nearest fixed star is to the sun, we should have to place our fleet in the middle of the Pacific Ocean, while the distance of the main shore of the starry universe would be so immense that the whole surface of the earth would be far too small to hold the expanse of ocean needed to represent it!

From these general considerations we next proceed to recall some of the details of the system of worlds amid which we dwell. Besides the earth, the sun has seven other principal planets in attendance. These eight planets fall into two classes—the terrestrial planets and the major, or jovian, planets. The former class comprises Mercury, Venus, the earth, and Mars, and the latter Jupiter, Saturn, Uranus, and Neptune. I have named them all in the order of their distance from the sun, beginning with the nearest.

The terrestrial planets, taking their class name from terra, the earth, are relatively close to the sun and comparatively small. The major planets—or the jovian planets, if we give them a common title based upon the name of their chief, Jupiter or Jove—are relatively distant from the sun and are characterized both by great comparative size and slight mean density. The terrestrial planets are all included within a circle, having the sun for a center, about 140,000,000 miles in radius. The space, or gap, between the outermost of them, Mars, and the innermost of the jovian planets, Jupiter, is nearly two and a half times as broad as the entire radius of the circle within which they are included. And not only is the jovian group of planets widely separated from the terrestrial group, but the distances between the orbits of its four members are likewise very great and progressively increasing. Between Jupiter and Saturn is a gap 400,000,000 miles across, and this becomes 900,000,000 miles between Saturn and Uranus, and more than 1,000,000,000 miles between Uranus and Neptune. All of these distances are given in round numbers.

Finally, we come to some very extraordinary worlds—if we can call them worlds at all—the asteroids. They form a third group, characterized by the extreme smallness of its individual members, their astonishing number, and the unusual eccentricities and inclinations of their orbits. They are situated in the gap between the terrestrial and the jovian planets, and about 500 of them have been discovered, while there is reason to think that their real number may be many thousands. The largest of them is less than 500 miles in diameter, and many of those recently discovered may be not more than ten or twenty miles in diameter. What marvelous places of abode such little planets would be if it were possible to believe them inhabited, we shall see more clearly when we come to consider them in their turn. But without regard to the question of habitability, the asteroids will be found extremely interesting.

In the next chapter we proceed to take up the planets for study as individuals, beginning with Mercury, the one nearest the sun.