The Plurality of Worlds

by William Whewell

THE NEBULÆ

1. I have attempted to show that, even if we suppose the other bodies of the universe to resemble the Earth, so far as to seem, by their materials, forms, and motions, no less fitted than she is to be the abodes of life; yet that, knowing what we do of man, we can believe that the Earth is tenanted by a race who are the special objects of God's care. Even if the tendency of the analogies of creation were, to incline us to suppose that the other planets are as well suited as our globe, to have inhabitants, still it would require a great amount of evidence, to make us believe that they have such inhabitants as we are; while yet such evidence is altogether wanting. Even if we knew that the stars were the centres of revolving systems, we should have an immense difficulty in believing that an Earth, with such a population as ours, revolves about any of them. If astronomy made a plurality of worlds probable, we have strong reasonings, drawn from other subjects, to think that the other worlds are not like ours.

2. The admirers of astronomical triumphs may perhaps be disposed to say, that when so much has been discovered, we may be allowed to complete the scheme by the exercise of fancy. I have attempted to show that we are not in such a state of ignorance, when we look at other relations of the earth and of man, as to allow us to do this. But now we may go a little onwards in our argument; and may ask, whether Astronomy really does what is here claimed for her:—whether she carries us so securely to the bounds of the visible universe, that our Fancy may take up the task, and people the space thus explored:—whether the bodies which Astronomy has examined, be really as fitted as our Earth, to sustain a population of living things:—whether the most distant objects in the universe do really seem to be systems, or the beginnings of systems:—whether Astronomy herself may not incline in favor of the condition of man, as being the sole creature of his kind?

3. In making this inquiry, it will of course be understood, that I do so with the highest admiration for the vast discoveries which Astronomy has really made; and for the marvellous skill and invention of the great men who have, in all ages of the world, and not least, in our time, been the authors of such discoveries. From the time when Galileo first discovered the system of Jupiter's satellites, to the last scrutiny of the structure of a nebula by Lord Rosse's gigantic telescope, the history of the telescopic exploration of the sky, has been a history of genius felicitously employed in revealing wonders. In this history, the noble labors of the first and the second Herschel relative to the distribution of the fixed stars, the forms and classes of nebulæ, and the phenomena of double stars, especially bear upon our present speculations; to which we may add, the examination of the aspect of each planet, by various observers, as Schroeter, and of the moon by others, from Huyghens to Mädler and Beer. The achievements which are most likely to occur to the reader's mind are those of the Earl of Rosse; as being the latest addition to our knowledge, and the result of the greatest instrumental powers. By the energy and ingenuity of that eminent person, an eye is directed to the heavens, having a pupil of six feet diameter, with the most complete optical structure, and the power of ranging about for its objects over a great extent of sky; and thus the quantity of light which the eye receives from any point of the heavens is augmented, it may be, fifty thousand times. The rising Moon is seen from the Observatory in Ireland with the same increase of size and light, as if her solid globe, two thousand miles in diameter, retaining all its illumination, really rested upon the summits of the Alps, to be gazed at by the naked eye. An object which appears to the naked eye a single star, may, by this telescope, so far as its power of seeing is concerned, be resolved into fifty thousand stars, each of the same brightness as the obvious star. What seems to the unassisted vision a nebula, a patch of diluted light, in which no distinct luminous point can be detected, may, by such an instrument, be discriminated or resolved into a number of bright dots; as the stippled shades of an engraving are resolved into dots by the application of a powerful magnifying glass. Similar results of the application of great telescopic power had of course been attained long previously; but, as the nature of scientific research is, each step adds something to our means of knowledge; and the last addition assumes, includes, and augments the knowledge which we possessed before. The discussions in which we are engaged, belong to the very boundary region of science;—to the frontier where knowledge, at least astronomical knowledge, ends, and ignorance begins. Such discoveries, therefore, as those made by Lord Rosse's telescope, require our special notice here.

4. We may begin, at what appears to us the outskirts of creation, the Nebulæ. At one time it was conceived by astronomers in general, that these patches of diffused light, which are seen by them in such profusion in the sky, are not luminous bodies of regular terms and definite boundaries, apparently solid, as the stars are supposed to be; but really, as even to good telescopes many of them seem, masses of luminous cloud or vapor, loosely held together, as clouds and vapors are, and not capable by any powers of vision of being resolved into distinct visible elements. This opinion was for a time so confidentially entertained, that there was founded upon it an hypothesis, that these were gaseous masses, out of which suns and systems might afterwards be formed, by the concentration of these luminous vapors into a solid central sun, more intensely luminous; while detached portions of the mass, flying off, and cooling down so as to be no longer self-luminous, might revolve round the central body, as planets and satellites. This is the Nebular Hypothesis, suggested by the elder Herschel, and adopted by the great mathematician Laplace.

5. But the result of the optical scrutiny of the nebulæ by more modern observers, especially by Lord Rosse in Ireland, and Mr. Bond in America, has been, that many celestial objects which were regarded before as truly nebulous, have been resolved into stars; and this resolution has been extended to so many cases of nebulæ, of such various kinds, as to have produced a strong suspicion in the minds of astronomers that all the nebulæ, however different in their appearance, may really be resolved into stars, if they be attacked with optical powers sufficiently great.

6. If this were to be assumed as done, and if each of the separate points, into which the nebulæ are thus resolved, were conceived to be a star, which looks so small only because it is so distant, and which really is as likely to have a system of planets revolving about it, as is a star of the first magnitude:—we should then have a view of the immensity of the visible universe, such as I presented to the reader in the beginning of this essay. All the distant nebulæ appear as nebulæ, only because they are so distant; if truly seen, they are groups of stars, of which each may be as important as our sun, being, like it, the centre of a planetary system. And thus, a patch of the heavens, one hundredth or one thousandth part of the visible breadth of our sun, may contain in it more life, not only than exists in the solar system, but in as many such systems as the unassisted eye can see stars in the heavens, on the clearest winter night.

7. This is a stupendous view of the greatness of the creation; and, to many persons, its very majesty, derived from magnitude and number, will make it so striking and acceptable, that, once apprehended, they will feel as if there were a kind of irreverence in disturbing it. But if this view be really not tenable when more closely examined, it is, after all, not wise to connect our feelings of religious reverence with it, so that they shall suffer a shock when we are obliged to reject it. I may add, that we may entertain an undoubting trust that any view of the creation which is found to be true, will also be found to supply material for reverential contemplation. I venture to hope that we may, by further examination, be led to a reverence of a deeper and more solemn character than a mere wonder at the immensity of space and number.

8. But whatever the result may be, let us consider the evidence for this view. It assumes that all the Nebulæ are resolvable into stars, and that they appear as nebulæ only because they are more distant than the region in which they can appear as stars. Are there any facts, any phenomena in the heavens, which may help us to determine whether this is a probable opinion?