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Once a Week (magazine)/Series 1/Volume 9/Are the planets inhabited?




Few questions can be started more curious or more interesting than that which relates to the existence or non-existence of life on the other planets of our solar system. Arguments on both sides have been urged with more or less ability; the negative being chiefly based on the assumption that they are unfitted, for physical reasons, to be the home of beings organised as we are. Though there is in reality no reason why living beings differently organised to ourselves should not inhabit those worlds, we propose to show that an inhabitant of the planet we occupy, might, if transported hence to one of them, be capable of continued existence there with just that slight modification which would grow out of a change of condition. As their adaptability for habitation must depend to a great extent on the matter of which they are composed, it is worth while stating the hypothesis which we conceive to be the most plausible as to the mode in which they were formed—an hypothesis, be it here remarked, which agrees, on the whole, with the opinions maintained by Sir D. Brewster and other high authorities.

The idea that the sun is an incandescent mass, seems to be confirmed by the recent experiments of Kirchoff, Bunsen, and others; and very wonderful indeed are the inferences which flow from their discoveries. We know that the sun revolves on its axis in a certain period, and appearances indicate that the same results follow from this in his case, as regards the regular set of currents of air, as on the earth. The larger and brighter masses of cloud are heaped together more thickly on either side of a band running across the sun’s disc than elsewhere, owing, as is supposed, to currents analogous to our trade winds. They are also observed to collect round the huge dark spots so frequently visible on the sun’s disc. The cause of these spots cannot be explained, but when numerous they do certainly affect the amount of light transmitted to us, as the experiments of Secchi show the light emitted from a spot near the centre of the sun does not exceed in quantity that which flows from the edge of the disc, where the luminosity is least, and from whence it goes on increasing towards the centre. But these masses of cloud are far from being of the innocent nature of those which float in our atmosphere. Instead of being particles of water, they are, there is good reason to believe, formed of metallic vapours, which, if they descend at all, pour down on the body of the sun in a fiery shower with a force, compared with which our tropical rains are light as falling dew. The mass of which the sun is composed is so enormous that the mind cannot form the faintest conception of the period which must elapse before its fires are extinguished and it disappears from the firmament, as other luminaries have done before it. Nor would it be possible, even if its combustion were more rapid than it is, to perceive any diminution in its dimensions, though the most careful observations were continued through successive generations. But that which generations could not perceive may well have taken place for all that, and it is easy to imagine that there was a time when in a nebulous state it filled the whole space included within the orbit of Neptune. Its revolution on its axis would cause the denser particles of which it was composed to fly outwards, and a ring would be formed which, by the dissipation of heat in space would probably contract and fracture, and the fractured portions may then have coalesced and assumed the form of a globe, retained in its orbit by the attraction of the mass of matter from which it had separated, and rotating on its axis in the same way as its constituent particles had done when it formed part of the parent body, or it may be that this continuous rotation may be due, as has been asserted, to the effects of electricity; a theory which was promulgated some years since in this country, though it has recently been revived in France, and spoken of as something quite novel.

Assuming the above theory to be true, the same process would be repeated as the sun continued to revolve and scatter its heat through space, and the planet Uranus would be formed. A repetition of it would produce Saturn, next Jupiter, then the huge planet, or the bodies that may once have been one, the Asteroids; then Mars, the Earth, Venus, and Mercury in succession, and, possibly, another planet within the orbit of Mercury, which from its proximity to that luminary is invisible to us. Assuming that we have now positive evidence that all the metals with which we are acquainted exist in a state of combustion in the sun’s atmosphere, and having regard to the physical appearances discernible on the planets belonging to our system, it may fairly be inferred upon this principle that they are all composed of similar substances. The question of their adaptability to the residence of organised beings, not differing essentially from ourselves, therefore would rest on their bulk, on the amount of light and heat they receive from the central orb, and on the presence of an atmosphere.

First, as regards their bulk. The weight of an object on the Earth is in proportion to the density of the globe and the distance of the object from its centre. The same holds good with respect to Jupiter, which is thirteen hundred and thirty times larger; and supposing the densities of both to be alike, the consequence would be that any object whatever, whether a man, a tree, or one of the pyramids, transported hence to that planet would crumble to pieces under the force of the attraction. But the densities of the planets are not equal. Taking first the exterior planet, Neptune, it is found that, bulk for bulk, its weight as compared with the Earth is as one to six, or nearly that of water, which is five and a half times lighter than the solid matter contained in our globe. Thus, though the bulk of Neptune is 107 times that of the Earth, its relative lightness, combined with the fact that an object on its surface is nearly five times more distant from its centre, would cause bodies to weigh nearly the same there as here. We will make this matter a little clearer to those who have not considered the subject.

If the density of Neptune were the same as the earth, bodies placed at the same distance from its centre would weigh 107 times heavier than here. But as the weight of a body, or the force with which it is drawn towards the centre of the globe[1] on which it rests, is diminished in proportion as it is more distant from the centre of attraction, a deduction would have to be made on this account, so that a man who weighed 150 lbs. on the Earth would actually weigh only 700 lbs. on the surface of Neptune. This is calculated on the supposition that the density of Neptune is the same as that of the planet we inhabit. The fact, however, is, as we have already said, that in consequence of the lightness of Neptune as compared with the Earth, a man going hence to that globe would be able to move with the same facility. As regards Uranus, though its dimensions are eighty-two times that of the earth, its weight, as compared with it, bulk for bulk, is not greater than that of Neptune, that is to say, as one to six, or a little less than water. The same powers of locomotion would therefore suffice if a man were transported to Uranus. Saturn is comparatively much lighter than either of the planets previously mentioned; they are of nearly the same weight as a globe of water would be, whereas Saturn is lighter in the proportion of one to one-and-a-half. But as its volume is 857 times that of the Earth, the actual weight of a man on its surface would be somewhat greater than on this globe, but not in a degree capable of impeding his movements to any serious extent.

The enormous dimensions of Jupiter, notwithstanding its lightness as compared bulk for bulk with the Earth, would render a residence on it irksome, though not impossible, without an increase of muscular power. On Mars half the strength we possess would be sufficient to enable us to move about and supply our wants with facility. The same may be said of Mercury: and as regards the only other planet of our system not yet mentioned, Venus, we should there be unconscious of having changed our habitation, as far as bodily strength is concerned.

We do not see, in the lightness of the majority of the planets as compared with our own globe, any ground for the inference that they are thereby unfit for human habitation, since lightness is quite compatible with solidity. The other objections that might be urged are, that there would be an insufficient supply of light and heat to support life on planets revolving at such an enormous distance from the central orb whence that light and heat emanates. At the first glance it would seem that if we have not a superabundance of either it must follow that Jupiter, which is five times more distant, must be deficient in both; and that this deficiency must go on increasing in an eminent degree as we recede to Saturn, which is nine times, to Uranus, which is eighteen times, and to Neptune, which is twenty-eight times more distant than the Earth. But a little consideration will show that, though regarded with our organs of vision, the sun, seen from Neptune, would appear of about the same diameter as a bright star, an enlargement of the pupils would cause objects on its surface to appear as brilliantly illuminated as on the Earth; the same result would be produced if the retina were rendered more sensible, either of which modifications might be produced without any alteration in the structural organisation of the eye.

But the most important question of all is: Have the other planets an atmosphere resembling ours? If they have not, it is clear that, however closely they may resemble the Earth in other respects, they cannot be inhabited by beings like us. If we were deprived of our atmosphere we know that there could be no clouds, no gradual passage from light to darkness; objects would be strongly lighted or in deep shadow, the sun would be a brilliant object, but the firmament would appear black, and dotted with stars; there would be neither life nor sound, and the Earth would circle round the Sun, a frozen ball, devoid of everything which would render life on it agreeable, even if it were possible.

Observations that have been made, establish conclusively the fact that other planets are enveloped in atmospheres. The two planets most favourably situated with respect to us for telescopic examinations, are Venus, Jupiter, and Mars. Surrounding Venus, we perceive what many astronomers consider to be a thick atmosphere,—so dense indeed that the twilight has been perfectly distinguished there; and this, together with its position, and the masses of cloud which float in it, denoting the presence of water, render the discovery of anything relative to the configuration of its surface highly improbable, beyond the fact that it has its chains of mountains, resembling those on the Earth. In the case of Mars, we are able to go beyond this. With a telescope possessing the requisite power, we can trace the boundaries of oceans and continents, and even the snow which lies at its polar circles, and the extent to which it is dissolved by the summer sun. By means of the lights and shadows on its surface, the fact that it rotates on its axis in as nearly as possible the same time as the Earth, has been proved; the same may also be said of the other planets, the differences in the time occupied in their respective revolutions being so trifling that it is not necessary to specify them.

Thus the existence of atmospheres round the other planets of our system being so highly probable, we have good grounds for believing that they are suited for the habitation of beings like ourselves. Objections on the ground of insufficient warmth are overruled at once: the degree of heat will be regulated by the density of the atmosphere. We know that we have only to ascend a mountain till we attain an altitude of 14,000 feet above the level of the sea, to find snow at the same time that the country at its foot is parched with heat, which is accounted for by the fact that the atmosphere is much more dense at the surface of the earth than at an elevation of three miles above it. If, then, our own experience enables us to prove that so slight a change of position in our atmosphere makes all the difference between life and death, surely no sane person will continue to urge the want of warmth in the more distant planets, as a reason for their being uninhabitable, when by a slight increase of density in their atmospheres their temperature would be raised to an equality with ours; moreover, we do not yet know that the heat we enjoy emanates entirely from the sun, or from the combined action of the sun’s rays and terrestrial agencies.

Having urged the preceding facts by way of proof that there is no essential difference between the physical condition of the Earth and that of the other planets of the system, it can be hardly necessary to pursue the argument of the extreme probability of their being inhabited by beings organised as we are.

The proofs that the globe we inhabit was expressly designed as a dwelling for us abound so thickly, that for any person to maintain that it was formed by a fortuitous concourse of atoms, is preposterous; and the evidences of design are not stronger in the case of the Earth than as regards other planets. We all remember how a distinguished German philosopher, who had been reflecting on this subject in his study, on entering his dining-room and perceiving a salad, suddenly exclaimed, “So, then, if lettuce, chervil, beetroot, and the other vegetables I see there, had been flying about in space with eggs, oil, and vinegar, they might at last have formed a salad!” “Yes,” answered his wife, “but not a salad like that before you.” The lady was undoubtedly right.—Simple as such a result might have appeared, the probabilities against the substances mixing themselves together in the proportions to form a good salad would puzzle a Quetelet to calculate.

If Chance had had anything to do with the formation of the Earth, there would be no reason why it should rotate on its axis in twenty-four hours, and yet a comparatively slight increase of that period might have rendered it uninhabitable. A very slight deviation from the actual inclination of its axis would have had a similar effect. Without an atmosphere we could not, of course, exist at all; but a very slight addition to one of the gases of which it is composed would destroy every living thing on the Earth’s surface, and its abstraction would reduce the globe to a mass of ashes. Everything, then, being so nicely adapted to the maintenance of life, and such an apparently trifling modification being capable of extinguishing it altogether, it is impossible to reflect on these matters without being driven to the conclusion that the Earth was formed expressly as a dwelling for us. And can we then, doubt that the same Creator who formed this globe created the others for a like purpose? That their inhabitants resemble us physically is only a reasonable supposition, considering the close resemblance of the different orbs: how far they may differ from us morally can only be imagined. George Leigh.


  1. It seems only justice to J. von Gumpach to mention here, that be has published a thick volume, in which he asserts that the opinion generally held that the form of the earth is similar to that of an orange is erroneous, and that tile real shape more resembles that of a lemon, and he argues that many of the wrecks which occur from a miscalculation of the position of the vessel arises from this erroneous belief.