LIFE IN OTHER WORLDS.—J. E. Gore
The question is often asked, Are the stars inhabited? To this we can confidently answer, No. The stars themselves are certainly not habitable by any forms of life with which we are familiar. That the stars are luminous incandescent bodies, similar to the sun, seems almost self-evident. That they shine by their own inherent light, and not by light reflected from another body, like the planets of the Solar System, is a fact which scarcely needs demonstration. There are no bright objects near them from which they could derive their light, and they are too far from the sun to obtain any illumination from that source. But if any proofs were necessary, we have the evidence of the spectroscope, which shows unmistakably that their light emanates from incandescent bodies. Many of the stars show spectra very similar to that of the sun. The light of others, although differing somewhat in quality when analyzed by the prism, indicates clearly that they are at a very high temperature—in many cases, indeed, suggesting that they are actually hotter than the sun. It may be objected, however, that in the case of binary or revolving double stars, the smaller component may possibly shine by light reflected from the brighter star. Indeed, this has been suggested in the case of Sirius and its faint companion. But, if the companion of Sirius shone merely by reflected light from its primary, it would be much fainter than it is, and, indeed, would be utterly invisible in our largest telescopes. Further, in some double stars, spectroscopic observations suggest that the component stars have different spectra. This is, of course, conclusive evidence against the hypothesis of borrowed light; for were the smaller star to shine by reflected light from the larger, the spectra of both would be identical, as in the case of the sun and moon. We may therefore conclude that all the visible stars are suns, and totally unfit for the habitation of living creatures.
But may not the stars have planets revolving round them, forming solar systems similar to our own? As they are evidently suns shining by inherent light, may they not form centres of planetary systems? In the case of those stars having spectra differing from the solar spectrum, we can not speak with any confidence; but for those which show spectra similar to that of our sun, and having, therefore, probably a similar chemical constitution, the existence of planets revolving round them seems from analogy very probable. I refer to single stars, that is stars which have no telescopic close companion; for the double stars may, perhaps, form systems differently constituted. In any case these binary systems would not be strictly comparable with ours, for the sun is certainly a single star.
Whether systems of planets really revolve round the stars referred to, is a question which, unfortunately, can not be decided by observation. If a planet equal in size to the “giant planet,” Jupiter, were revolving round the nearest star—Alpha Centauri—at the same distance from that star that Jupiter is from the sun, it would be utterly invisible in our largest telescopes. The invisibility of planets circling round the stars is therefore no proof whatever of their non-existence. Each star of the solar type may possibly be attended by a retinue of planets which may, perhaps, remain forever invisible in the largest telescopes which man can construct. We can, therefore, draw our conclusions only from analogy. If other suns exist resembling our own sun in chemical constitution, which we know to be a fact, is it not reasonable to suppose that they also form centres of planetary systems similar to the Solar System?
“Consult with reason, reason will reply,
Each lucid point which glows in yonder sky,
Informs a system in the boundless space,
And fills with glory its appointed place;
With beams unborrowed brighten other skies,
And worlds to the unknown with heat and light supplies.”
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 suns revealed by the telescope. They must, therefore, have been formed for some other purpose.
On Laplace’s Nebular Hypothesis, the condensation of an original nebulous mass endowed with a motion of rotation would result not only in the formation of a sun, similar to ours, but also in a system of planets revolving round the central body. If, indeed, the primitive nebula had no rotation or motions of any kind, the result would be a sun without planets or satellites; but the motions with which all the stars seem to be animated lead us to suppose that this would be a case of very rare occurrence. 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.
This being surmised, let us consider the conditions necessary for the existence of life on these planets. There are various conditions which must be complied with before we can imagine life, as we know it, to be possible on any planet. Perhaps the most important of these is the question of temperature. We know that in the universe a great range of temperature exists, from the cold of interstellar space—estimated at about 460° below the freezing-point of water—to the intense heat which rages in the solar photosphere. In this long thermal scale life is, at least on the earth, restricted within rather narrow limits. Below a certain low temperature life can not exist. The point is, however, far above the temperature of space. On the other hand, above a certain high temperature—a low one, however, compared with the intense heat of the solar surface—life is also impossible, at least for highly organized beings like man and the larger animals. For minute microscopic organisms the scale may, perhaps, be somewhat extended; but even in its widest limits, the range of temperature within which life is possible is, so far as we know, certainly a narrow one.
For the support of life and vegetation, light is also necessary, for without it no flowers would bloom, nor corn grow and ripen to maturity. To obtain this supply of light and heat it is necessary that a life-bearing planet should revolve at a suitable distance from, and in a nearly circular orbit round, a central sun. These conditions, it is hardly necessary to say, are fulfilled in the case of the earth. Were we much nearer to the sun than we are, we should suffer from excessive heat, and were we much further away, we should probably perish from the cold. For this reason the existence of life on the other planets of the Solar System seems very doubtful. Mercury is probably too hot, and the other planets are certainly too cold, so far as heat from the sun is concerned, unless, indeed, their internal heat is sufficient to raise the temperature of their surface to a point sufficient for the maintenance of life. Indeed, there is good reason to suppose that in the planets Jupiter, Saturn, Uranus, and Neptune, this internal heat is still so great that life would be quite impossible on their surface. Venus, inside the earth’s orbit, and Mars, outside, are the two planets which seem to approach nearest to the required conditions. We know that both these planets possess atmospheres somewhat similar to ours, and, in Mars at least, land and water most probably exist on its surface. Venus is, of course, much hotter than the earth, and Mars much colder, but possibly the polar regions of Venus and the equatorial regions of Mars may form suitable abodes for some forms, at least, of animal and vegetable life.
Let us proceed, however, to consider some other conditions necessary for the existence of life on a planet. A suitable temperature is, of course, indispensable, but this is not all. There are other conditions which must be complied with. The planet must have a rotation on its axis, so that every portion shall in turn receive its due share of light and heat. Each point on its surface must have its day and night, the day for work and the night for rest. The axis of rotation must not lie in the plane of the planet’s orbit, but must have a suitable inclination, so that each hemisphere may enjoy its seasons, summer and winter, “seed-time and harvest,” in due course. Further the velocity of rotation on its axis must not be too rapid. If the earth rotated in a period of one and a quarter hours, bodies at the equator would have no weight, and life would be impossible in those regions.
The planet must also possess a mass sufficient to retain bodies on its surface by the force of gravity. In the case of very small bodies, such as the moons of Mars and some of the minor planets between Mars and Jupiter, objects thrown into the air would pass away into space never to return.
The planet should also have a mean density greater than that of water, otherwise the seas would possess no stability, and destructive waves would quickly destroy all life on its surface. All these conditions are fulfilled in the case of Mars as well as on the earth. In the planet Saturn, however, the density is less than that of water, and in Uranus and Neptune only slightly greater.
The planet must also possess a suitable atmosphere. This is an all-important condition for the support of animal life—at least for the existence of man and the higher orders of animals. This atmosphere must consist—so far as we know—of oxygen and nitrogen gases mechanically mixed in proper proportions, and with a small quantity of carbonic acid gas. Were the oxygen in smaller quantity than it exists in the earth’s atmosphere, life could not be supported. On the other hand, were it much in excess of its present amount, a fever would be produced in the blood which would very soon put an end to animal life. The presence of other gases in excessive quantities would also render the air unfit for breathing. We see, therefore, that a comparatively slight change in the composition of a planet’s atmosphere would—so far as our experience goes—render the planet uninhabitable by any of the higher forms of life with which we are familiar.
For the support of life on a planet, water is also absolutely necessary. Without this useful fluid the world would soon become a desert, and life and vegetation would speedily vanish from its surface.
Geological conditions must also be considered. It is clearly necessary for the welfare of human beings at least that the surface soil and rocks should contain coal, iron, lime, and other minerals, substances almost indispensable for the ordinary wants of civilized existence.
Nine Views of the Hour-Glass Sea on Mars
1, Nov. 26, 1864; 2, June 29, 1873; 3, Oct. 28, 1879; 4, June 2, 1888; 5, June 20, 1890; 6, Aug. 6, 1892; 7, Oct., 1894; 8, Dec. 3, 1896; 9, Dec. 7, 1896
That all or any of the conditions considered would be complied with in the case of a planet revolving round a star it is, of course, impossible to say. But when we find stars showing by their spectra that they contain chemical elements identical with those which exist in the sun and the earth, analogy would lead us to suppose that very possibly a planet resembling our earth may revolve round each of these distant suns. I say a planet, for evidently there would be only one distance from the central luminary—a distance depending on its size—at which the temperature necessary for the support of life would exist, as in the case of the earth, over the whole of the planet’s surface. For other planets of the stellar system, life would be, if it existed at all, most probably confined to restricted regions of the planet’s surface. There would, therefore, be in each system one planet, and only one, especially suitable for the support of animal life as we know it. This is with reference to light and heat. If the other conditions were not complied with, then life would probably not exist even on this one planet. In the case of a star larger than the sun, the planet should be placed at a greater distance than the earth is from the sun, but in this case the length of the year and the seasons would be longer than ours.
The star which more nearly resembles the sun in the character of the light which it emits is the bright star Capella. Arcturus has a somewhat similar spectrum. But these are probably suns of enormous size, if any reliance can be placed on the measures of their distance from the earth. Other bright stars with spectra of the solar type are Pollux, Aldebaran, Beta Andromedæ, Alpha Arietis, Alpha Cassiopeiæ, Alpha Cygni, and Alpha Ursæ Majoris. Another star is Eta Herculis. The magnitude of this star as measured with the photometer is about 3½. A parallax found by Bélopolsky and Wagner places it at a distance of 515,660 times the sun’s distance from the earth. If the sun were placed at this distance, I find that it would be reduced to a star of the third magnitude. This result would imply that Eta Herculis is a slightly smaller sun than ours; and a planet placed a little nearer to the star than the earth is to the sun might, perhaps, fulfil the conditions of a life-bearing world.
The number of stars visible in our largest telescopes is usually estimated at 100,000,000. Of these we may perhaps assume that 10,000,000 have a spectrum of the solar type, and therefore closely resemble our sun in their chemical constitution. If we suppose that only one in ten of these is similar in size to the sun, and has a habitable planet revolving round it, we have a total of 1,000,000 worlds in the visible universe fitted for the support of animal life.
We may therefore conclude, with a high degree of probability, that among the “multitudinous” stellar hosts there are probably many stars having life-bearing planets revolving round them.