The Plurality of Worlds

by William Whewell

4. But this is only the first step in the asserted revelations of astronomy. Some of the stars are, as we have said, planets of the kind just described. But these stars are a few only:—five, or at most six, of those visible to the unassisted eye of man. All the rest, innumerable as they appear, and numerous as they really are, are, it is found, objects of another kind. They are not, as the planets are, opaque globes, deriving their light from a sun, about which they circulate. They shine by a light of their own. They are of the nature of the sun, not of the planets. That they appear mere specks of light, arises from their being at a vast distance from us. At a vast distance they undoubtedly are; for even with our most powerful telescopes, they still appear mere specks of light;—mere luminous points. They do not, as the planets do, when seen through telescopes, exhibit to us a circular face or disk, capable of being magnified and distinguished into parts and features. But this impossibility of magnifying them by means of telescopes, does not at all make us doubt that they may be far larger than the planets. For we know, from other sources of information, that their distance is immensely greater than that of any of the planets. We can measure the bodies of the solar system;—the earth, by absolutely going round a part of it, or in other ways; the other bodies of the system, by comparing their positions, as seen from different parts of the earth. In this manner we find that the earth is a globe 8,000 miles in diameter. In this way, again, we find that the circle which the earth describes round the sun has, in round numbers, a radius about 24,000 times the earth's radius; that is, nearly a hundred millions of miles. The earth is, at one time, a hundred millions of miles on one side of the sun; and at another time, half a year afterwards, a hundred millions of miles on the other side. Of the bright stars which shine by their own light,—the fixed stars, as we call them, (to distinguish them from the planets, the wandering stars,)—if any one were at any moderate distance from us, we should see it change its apparent place with regard to the others, in consequence of our thus changing our point of view two hundred millions of miles: just as a distant spire changes its apparent place with regard to the more distant mountain, when we move from one window of our house to the other. But no such change of place is discernible in any of the fixed stars: or at least, if we believe the most recent asserted discoveries of astronomers, the change is so small as to imply a distance in the star, of more than two hundred thousand times the radius of the earth's orbit, which is, itself, as we have said, one hundred millions of miles.[1] This distance is so vastly great, that we can very well believe that the fixed stars, though to our best telescopes they appear only as points of light, are really as large as our sun, and would give as much light as he does, if we could approach as near to them. For since they are thus, the nearest of them, two hundred thousand times as far off as he is, even if we could magnify them a thousand times, which we can hardly do, they would still be only one two-hundredth of the breadth of the sun; and thus, still a mere point.

5. But if each fixed star be of the nature of the sun, and not smaller than the sun, does not analogy lead us to suppose that they have, some of them at least, planets circulating about them, as our sun has? If the Sun is the centre of the Solar System, why should not Sirius, (one of the brightest of the fixed stars,) be the centre of the Sirian System? And why should not that system have as many planets, with the same resemblances and differences of the figure, movements, and conditions of the different planets, as this? Why should not the Sirian System be as great and as varied as the solar system? And this being granted, why should not these planets be inhabited, as men have inferred the other planets of the solar system, as well as the earth, to be? And thus we have, added to the population of the universe of which we have already spoken, a number (so far as we have reason to believe) not inferior to the number of inhabitants of the solar system: this number being, according to all the analogies, very many fold that of the population of the whole earth?

And this is the conclusion, when we reason from one star only, from Sirius. But the argument is the same, from each of the stars. For we have no reason to think that Sirius, though one of the brightest, is more like our sun than any of the others is. The others appear less bright in various degrees, probably because they are further removed from us in various degrees. They may not be all of the same size and brightness; it is very unlikely that they are. But they may as easily be larger than the sun, as smaller. The natural assumption for us to make, having no ground for any other opinion, is, that they are, upon the average, of the size of our sun. On that assumption, we have as many solar systems as we have fixed stars; and, it may be, six or ten, or twenty times as many inhabited globes; inhabited by creatures of whom we must suppose, by analogy, that God is mindful, if he is mindful of us. The question recurs with overwhelming force, if we still follow the same train of reflection: "What is man, that God is mindful of him?"

6. But we have not yet exhausted the views which thus add to the force of this reflection. The fixed stars, which appear to the eye so numerous, so innumerable, in the clear sky on a moonless night, are not really so numerous as they seem. To the naked eye, there are not visible more than four or five thousand. The astronomers of Greece, and of other countries, even in ancient times, counted them, mapped them, and gave them names and designations. But Astronomy, who thus began her career by diminishing, in some degree, the supposed numbers of the host of heaven, has ended by immeasurably increasing them. The first application of the telescope to the skies discovered a vast number of fixed stars, previously unseen: and every improvement in that instrument has disclosed myriads of new stars, visibly smaller than those which had before been seen; and smaller and smaller, as the power of vision is more and more strengthened by new aids from art; as if the regions of space contained an inexhaustible supply of such objects; as if infinite space were strewn with stars in every part of it to which vision could reach. The small patch of the sky which forms, at any moment, the field of view of one of the great telescopes of Herschel, discloses to him as many stars, and those of as many different magnitudes, as the whole vault of the sky exhibits to the naked eye. But the magnifying power of such an instrument only discloses, it does not make, these stars. There appears to be quite as much reason to believe, that each of these telescopic stars is a sun, surrounded by its special family of planets, as to believe that Sirius or Arcturus is so. Here, then, we have again an extension, indefinite to our apprehension, of the universe, as occupied by material structures; and if so, why not by a living population, such as the material structures which are nearest to us support?

7. Even yet we have not finished the series of successive views which astronomers have had opened to them, extending more and more their spectacle of the fulness and largeness of the universe. Not only does the telescope disclose myriads of stars, unseen to the naked eye, and new myriads with each increase of the powers of the instrument; but it discloses also patches of light, which, at first at least, do not appear to consist of stars: Nebulæ, as they are called; bright specks, it might seem, of stellar matter, thin, diffused, and irregular; not gathered into regular and definite forms, such as we may suppose the stars to be. Every one who has noticed the starry skies, may understand what is the general aspect of such nebulæ, by looking at the milky way or galaxy, an irregular band of nebulous light, which runs quite round the sky; "A circling zone, powdered with stars;" as Milton calls it. But the nebulæ of which I more especially speak, are minute patches, discovered mainly by the telescope, and in a few instances only discernible by the naked eye. And what I have to remark especially concerning them at present is, that though to visual powers which barely suffice to discern them, they appear like mere bright clouds, patches of diffused starry matter; yet that, when examined by visual powers of a higher order, by more penetrating telescopes, these patches of continuous feeble light are, in many instances at least, distinguishable into definite points: they are found, in fact, to be aggregations of stars; which before appeared as diffused light, only because our telescopes, though strong enough to reveal to our senses the aggregate mass of light of the cluster, were not strong enough to enable us to discern any one of the stars of which the cluster consists. The galaxy, in this way, may, in almost every part, be resolved into separate stars; and thus, the multitude of the stars in the region of the sky occupied by that winding stream of light, is, when examined by a powerful telescope, inconceivably numerous.

8. The small telescopic nebulæ are of various forms; some of them may be in the shape of flat strata, or cakes, as it were, of stars, of small thickness, compared with the extent of the stratum. Now, if our sun were one of the individuals of such a stratum, we, looking at the stars of the stratum from his neighborhood, should see them very numerous and close in the direction of the edge of the stratum, and comparatively few and rare in other parts of the sky. We should, in short, see a galaxy running round the sky, as we see in fact. And hence Sir William Herschel has inferred, that our sun, with its attendant planets, has its place in such a stratum; and that it thus belongs to a host of stars which are, in a certain way, detached from the other nebulæ which we see. Perhaps, he adds, some of those other nebulæ are beds and masses of stars not less numerous than those which compose our galaxy, and which occupy a larger portion of the sky, only because we are immersed in the interior of the crowd. And thus, a minute speck of nebulous light, discernible only by a good telescope, may contain not only as many stars as occupy the sky to ordinary vision, but as many as is the number into which the most powerful telescope resolves the milky light of the galaxy. And of such resolvable nebulæ the number which are discovered in the sky is very great, their forms being of the most various kind; so that many of them may be, for aught we can tell, more amply stocked with stars than the galaxy is. And if all the stars, or a large proportion of the stars, of the galaxy, be suns attended by planets, and these planets peopled with living creatures, what notion must we form of the population of the universe, when we have thus to reckon as many galaxies as there are resolvable nebulæ! the stock of discoverable nebulæ being as yet unexhausted by the powers of our telescopes; and the possibility of resolving them into stars being also an operation which has not yet been pursued to its limit.

9. For, (and this is the last step which I shall mention in this long series of ascending steps of multitude apparently infinite,) it now begins to be suspected that not some nebulæ only, but all, are resolvable into separate stars. When the nebulæ were first carefully studied, it was supposed that they consisted, as they appeared to consist, of some diffused and incoherent matter, not of definite and limited masses. It was conceived that they were not stars, but Stellar Matter in the course of formation into stars; and it was conceived, further, that by the gradual concentration of such matter, whirling round its centre while it concentrated, not only stars, that is, suns, might be formed, but also systems of planets, circling round these suns; and thus this Nebular Hypothesis, as it has been termed, gave a kind of theory of the origin and formation of systems, such as the solar system. But the great telescope which Lord Rosse has constructed, and which is much more powerful than any optical instrument yet fabricated, has been directed to many of the nebulæ, whose appearance had given rise to this theory; and the result has been, in a great number of cases, that the nebulæ are proved to consist entirely of distinct stars; and that the diffused nebulous appearance is discovered to have been an illusion, resulting from the accumulated light of a vast number of small stars near to each other. In this manner, we are led to regard every nebula, not as an imperfectly formed star or system, but as a vast multitude of stars, and, for aught we can tell, of systems; for the apparent smallness and nearness of these stars are, it is thought, mere results of the vast distance at which they are placed from us. And thus, perhaps, all the nebulæ are, what some of them seem certainly to be, so many vast armies of stars, each of which stars, we have reason to believe, is of the nature of our sun; and may have, and according to analogy has, an accompaniment of living creatures, such as our sun has, certainly on the earth, probably, it is thought, in the other planets.

10. It is difficult to grasp, in one view, the effect of the successive steps from number to number, from distance to distance, which we have thus been measuring over. We may, however, state them again briefly, in the way of enumeration.

From our own place on the earth, we pass, in thought, as a first step, to the whole globe of the Earth; from this, as a second step, to the Planets, the other globes which compose the Solar System. A third step carries us to the Fixed Stars, as visible to the naked eye; very numerous and immensely distant. The transition to the Telescopic Stars makes a fourth step; and in this, the number and the space are increased, almost beyond the power of numbers to express how many there are, and at what distances. But a fifth step:—perhaps all this array of stars, obvious and telescopic, only make up our Nebula; while the universe is occupied by other Nebulæ innumerable, so distant that, seen from them, our nebula, though including, it may be, stars of the 20th magnitude, which may be 20 times or 2,000 times more remote than Sirius, would become a telescopic speck, as their nebulæ are to us.