These are the remarks of Ferguson(74). One of our latest astronomers expresses himself to the same purpose.
(74) Astronomy, Section 22.
"We have no argument against the planets being inhabited by rational beings, and consequently by witnesses of the creator's power, magnificence and benevolence, unless it be said that some are much nearer the sun than the earth is, and therefore must be uninhabitable from heat, and those more distant from cold. Whatever objection this may be against their being inhabited by rational beings, of an organisation similar to those on the earth, it can have little force, when urged with respect to rational beings in general.
"But we may examine without indulging too much in conjecture, whether it be not possible that the planets may be possessed by rational beings, and contain animals and vegetables, even little different from those with which we are familiar.
"Is the sun the principal cause of the temperature of the earth? We have reason to suppose that it is not. The mean temperature of the earth, at a small depth from the surface, seems constant in summer and in winter, and is probably coeval with its first formation.
"At the planet Mercury, the direct heat of the sun, or its power of causing heat, is six times greater than with us. If we suppose the mean temperature of Mercury to be the same as of the earth, and the planet to be surrounded with an atmosphere, denser than that of the earth, less capable of transmitting heat, or rather the influence of the sun to extricate heat, and at the same time more readily conducting it to keep up an evenness of temperature, may we not suppose the planet Mercury fit for the habitation of men, and the production of vegetables similar to our own?
"At the Georgium Sidus, the direct influence of the sun is 360 times less than at the earth, and the sun is there seen at an angle not much greater than that under which we behold Venus, when nearest. Yet may not the mean temperature of the Georgium Sidus be nearly the same as that of the earth? May not its atmosphere more easily transmit the influence of the sun, and may not the matter of heat be more copiously combined, and more readily extricated, than with us? Whence changes of season similar to our own may take place. Even in the comets we may suppose no great change of temperature takes place, as we know of no cause which will deprive them of their mean temperature, and particularly if we suppose, that on their approach towards the sun, there is a provision for their atmosphere becoming denser. The tails they exhibit, when in the neighbourhood of the sun, seem in some measure to countenance this idea.
"We can hardly suppose the sun, a body three hundred times larger than all the planets together, was created only to preserve the periodic motions, and give light and heat to the planets. Many astronomers have thought that its atmosphere only is luminous, and its body opake, and probably of the same constitution as the planets. Allowing therefore that its luminous atmosphere only extricates heat, we see no reason why the sun itself should not be inhabited(75)."
(75) Brinkley, Elements of Astronomy, Chap. IX.
There is certainly no end to the suppositions that may be made by an ingenious astronomer. May we not suppose that we might do nearly as well altogether without the sun, which it appears is at present of little use to us as to warmth and heat? As to light, the great creator might, for aught we know, find a substitute; feelers, for example, endued with a certain acuteness of sense: or, at all events, the least imaginable degree of light might answer every purpose to organs adapted to this kind of twilight. In that way the inhabitants of the Georgium Sidus are already sufficiently provided for; they appear to have as little benefit of the light as of the heat of the sun. How the satellites of the distant planets are supplied with light is a mystery, since their principals have scarcely any. Unless indeed, like the sun, they have a luminous atmosphere, competent to enlighten a whole system, themselves being opake. But in truth light in a greater or less degree seems scarcely worthy of a thought, since the inhabitants of the planet Mercury have not their eyes put out by a light, scarcely inferior in radiance to that which is reflected by those plates of burning brass, with which tyrants in some ages were accustomed to extinguish the sense of vision in their unfortunate victims. The comets also must be a delectable residence; that of 1680 completing its orbit in 576 years, and being at its greatest distance about eleven thousand two hundred millions of miles from the sun, and at its least within less than a third part of the sun's semi-diameter from its surface(76). They must therefore have delightful vicissitudes of light and the contrary; for, as to heat, that is already provided for. Archdeacon Brinkley's postulate is, that these bodies are "possessed by rational beings, and contain animals and vegetables, little different from those with which we are familiar."