HABITABILITY OF WORLDS. —
I. ACCORDING TO SCIENCE
By habitability of worlds is meant a complex of conditions suitable for the forms of life existing on Earth, without denying the absolute possibility of forms of life adapted to biospheres entirely different from the terrestrial.A first examination concerns our immediate surroundings: the solar system. It can be said that astronomers generally agree in holding that the planets with their satellites, in their present conditions, are not habitable by organized beings such as those on Earth. Mercury has no atmosphere, and since it always turns the same face toward the Sun, the face at the center has a temperature of about 350° C., while the opposite face is in perpetual night and a cold approaching absolute zero. Venus is covered with dense clouds of carbonic acid, while the spectroscope has shown the absence of water vapor and oxygen. Since rotation is very probably very slow, the solar rays penetrating to the surface cause a temperature in the center like that of boiling water. The absence of water vapor in the atmosphere is a sign that there are no oceans, and probably not even water. Now a planet without water cannot be suitable ground for life, and the lack of oxygen and the abundance of carbonic acid confirm this opinion. On Mars conditions are found that are more similar to those on Earth. The average temperature of —40° C. rises at the equator, in summer, to about ten degrees above zero, while during winter nights it drops to —90° C. But the atmosphere, very thin, contains a scant amount of oxygen and carbonic acid, and perhaps not even that; therefore vegetation comparable to that on Earth is not possible there, but at most some moss-like vegetation, of which the change from green to brown observed in some dark spots with the changing seasons of Mars, toward the summer solstice, might be an indication. It seems, however, that in the spectrum of the dark spots the typical reflection of terrestrial vegetation is lacking, namely, chlorophyll. In short, not even Mars presents ground suitable for higher forms of life. The Moon has no air and no water, and the temperature varies between 120° C. and —80° C. On Jupiter, with its atmosphere of methane and ammonia without oxygen, without carbonic acid or moisture, and with a temperature of —130° C., every form of life is absolutely excluded. The same holds for Saturn, Uranus, Neptune, Pluto, whose temperatures are even lower than those of Jupiter. One of the main constituents of their atmospheres is methane, to a lesser degree ammonia, which, frozen at those extremely low temperatures, disappears from the atmosphere. The temperature of Neptune is so low that nitrogen can exist there only in the solid state.
As for the existence of planets in the celestial space outside the solar system, we have no information. The stars, even the nearest ones, are so distant from us that any possible planets, even of the size of Jupiter, lie beyond the reach of the most powerful telescopes.
Therefore the habitability of worlds outside the solar system is a matter of pure speculation.
II. ACCORDING TO RELIGION
As regards the question of the habitability of worlds, Catholic doctrine affirms nothing categorically; in this respect, therefore, full freedom of opinion and discussion remains. In fact, both Catholic philosophers and scientists are found who incline to consider inhabited, if not all, at least some of the stars; others who, while denying habitability for the present, consider it probable at some more or less distant future time, when the individual stars will have reached the degree of evolution and transformation required to satisfy the needs of man with respect to atmosphere, environment, nourishment, etc. Just as it happened for the Earth, which did not begin to be inhabited by man until after many and many centuries of existence (cf. Secchi, Denza, Pohle, Müller, in the works cited in the bibliography).The question, however, has been transferred to the religious terrain, where arguments for or against the habitability of worlds have been sought. Some, for example, C. Flammarion (op. cit., pp. 343-46), assert that the Church would never admit the plurality of inhabited worlds, because the mysteries of the Incarnation and the Redemption would be at stake. Of the Incarnation, because it would no longer be possible to say that the Word became incarnate, infinitely lowering and almost annihilating Himself, according to the phrase of St. Paul (Phil. 2:7), for a handful of men scattered on Earth, an atom of dust in comparison with the other innumerable stars, and who would no longer even have the privilege of being the center of our solar system. The handful of earthly men would dissolve into the multitude of astral inhabitants, and the mystery would lose much of its grandeur, its force, and its greatness of love.
It can be answered that the mystery of the Incarnation would always remain the same incomprehensible abyss of humiliation and self-giving, even if the Word had become incarnate for an immense multitude of men, because “all nations are before Him as if they were not” (Is. 40:17). What makes the mystery of humiliation and love great is not the greater or lesser number of men who have drawn or draw or will draw benefit from it, but the fact itself of a God who lowers Himself to take the form of man. Even if it were only one man, the mystery would lose nothing; indeed, it would gain, because it would be proof of how immense is the divine love for man.
Nor would the plurality of inhabited worlds endanger the dogma of the Redemption. It is objected that historically Jesus Christ sacrificed Himself on Earth; now if the other stars were inhabited, how could those men have known the mystery and participated in its effects? Here again the answer is simple: even supposing that among the inhabitants of the other worlds there had also been a defection from obedience to God, as happened with Adam, how could it be asserted that the hand of almighty God was shortened so as not to find a way of redemption for them as well? All the more so since “Deus per suam omnipotentem virtutem poterat humanam naturam multis aliis modis reparare” (Sum. Theol., 1a, q. 1, a. 2) even with respect to the Earth.
Moreover, it is said: whoever reads the story of creation in the book of Moses or the psalms of David or the inspired words of other sacred writers remains with the impression that earthly man is the true moral center of the entire universe and that all the rest—the sun, the moon, the firmament—were created for man. To populate the stars with other men seems to dethrone earthly man and deny him the importance granted him by God. It must first be answered that the revelation contained in Holy Scripture does not aim to instruct on cosmological and astronomical doctrines, and therefore the inspired author adapts himself, in speaking, to the doctrines current in his time. On the other hand, earthly man, even in the hypothesis of millions of other worlds, ennobled by the presence of man, would not in any way lose the outstanding privilege accorded him of having been raised to a sublime dignity by the fact that the Word became man on Earth and assumed the nature of earthly man.
Finally, it is objected that some ecclesiastical writers in past times would not even admit the existence of the antipodes on Earth; still less would they be willing to admit other men on the stars. The fact that
Opponents to the existence of the antipodes were indeed in error; but their reasoning did not derive from consideration of the antipodes themselves, rather from the supposition that such men could not be descendants of Adam. This supposition was false and had no true support in Catholic doctrine (v. ANTIPODI).
On the other hand, some would find in Holy Scripture allusions favorable to the hypothesis of the plurality of worlds, referring to: 1) the parable of the 99 sheep left by the Good Shepherd to seek the one that was lost. Jesus would have left the inhabitants of the stars, presumed innocent, to descend and save the sinful inhabitants of the earth; 2) the words of Jesus: “I have other sheep that are not of this fold… these also I must gather” (Jn 10:16). The “other sheep” would be the inhabitants of the stars, of whom Jesus Christ is also King; 3) the phrase of St Paul: “It pleased God… to reconcile all things in him, whether on earth or in heaven” (Col 1:20); since the good angels were certainly not redeemed by Jesus Christ, the phrase “those in heaven” would refer to the humanity of the stars; 4) and finally to the liturgical hymn (by Venantius Fortunatus): “Terra, pontus, astra, mundus quo lavantur flumine!” (In the hymn Lustra sex qui iam peregit, for Lauds in Passiontide); the redeeming blood would have regenerated not only the earth, that is, the terrestrial inhabitants, but also the heavens, namely the celestial inhabitants.
To all these citations it must be replied: these and other similar texts prove nothing regarding the question of the plurality of worlds; their primary meaning, according to Church doctrine, is otherwise. It may nevertheless be said, with some theologians, that if scientific demonstration of the plurality of inhabited worlds were established, such texts would also allude in veiled terms to the existence of these celestial inhabitants.
In conclusion: the day on which science succeeds in proving that even on other planets, near or far, and on other stars there are rational beings like ourselves, philosophy will explain the origin of those men in the same way as for terrestrial men, by appealing to the extension of causality, which postulates a creating Being. And theology will invite us to magnify still more the greatness, goodness, and infinite generosity of God.