ABITABILITÀ DEI MONDI. - 1. ACCORDING TO SCIENCE. - By h. of the worlds is meant a complex of conditions suited to the forms of life existing on earth, without denying the absolute possibility of forms of life suited to biospheres totally different from the terrestrial one.
A first examination concerns our immediate surroundings: the solar system. It may 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 terrestrials. Mercury has no atmosphere, and always turning the same face toward the sun, the face at the center has a temperature of about 350° C., while on the opposite face perpetual night reigns and a cold down almost to absolute zero. Venus is covered with dense clouds of carbonic acid, while the spectroscope has shown the absence of water vapor and of oxygen. Since its rotation is very probably quite slow, the sun's rays penetrating to the surface cause there, at the center, a temperature 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 a terrain suited to life, and the lack of oxygen and the abundance of carbonic acid confirm this view. On Mars conditions are found more similar to terrestrial ones. The mean temperature of — 40° C. rises at the equator, in summer, to about ten degrees above zero, while during winter nights it descends to — 90° C. But the atmosphere, quite thin, contains a scanty quantity of oxygen and of carbonic acid, and perhaps not even this: therefore a vegetation comparable to the terrestrial one is not possible there, but at most some mossy vegetation, of which the change from green to brown observed in certain dark spots with the varying of the Martian season, toward the summer solstice, would be an indication. It seems, however, that in the spectrum of the dark spots the reflection typical of terrestrial vegetation, that is, chlorophyll, is lacking. In short, not even Mars presents a terrain suited to a higher life. The Moon has no air and not even water, and the temperature varies between 120° C. and — 80° C. On Jupiter, with its mephitic atmosphere of methane and ammonia without oxygen, without carbonic acid or moisture, with a temperature of — 130° C., every form of life is altogether excluded. The same holds for Saturn, Uranus, Neptune, Pluto, whose temperatures are still much lower than those of Jupiter. One of the principal constituents of their atmospheres is methane, and to a lesser degree ammonia, which, frozen at those very 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 celestial space outside the solar system, we have no information. The stars, even the nearest, are so distant from us that any planets, even of the dimensions of Jupiter, fall outside the reach of the most powerful telescopes.
Therefore the h. of worlds outside the solar system is a question of pure fantasy.
Bibliography4 works
Giovanni Stein
2. ACCORDING TO RELIGION
On the question of the h. of the worlds Catholic doctrine affirms nothing categorical; under this aspect, therefore, freedom of opinion and of discussion remains complete. There are found, in fact, both Catholic philosophers and scientists who incline to consider inhabited, if not all, at least a part of the stars; others who, denying h. for the present, nonetheless hold it probable in a more or less near future, when the individual stars will have reached the degree of evolution and transformation required to satisfy man's needs as regards atmosphere, environment, nourishment, etc. Precisely as happened for the earth, which did not begin to harbor 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 carried onto religious ground, where arguments for or against the h. of the 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 of the Redemption would be at stake. Of the Incarnation, because it could no longer be said that the Word became incarnate, humbling Himself infinitely and almost annihilating Himself, according to the phrase of St. Paul (Phil. 2, 7), for a handful of men, scattered over the earth, an atom of dust in comparison with the other innumerable stars, and which no longer even has the privilege of being the center of our solar system. The handful of terrestrial 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.
One may answer that the mystery of the Incarnation would always remain the same incomprehensible abyss of humiliation and of self-giving, even if the Word had become incarnate for an immense multitude of men, because "all the nations are before Him as if they were not" (Is. 40, 17). What makes the mystery of humiliation and of love great is not the greater or lesser number of men who drew or draw or will draw fruit from it, but the fact in itself of a God who stoops to take the form of man. Even if it were a question of a single man, the mystery would lose nothing; one should rather say that it would gain, because it would be the proof of how immense is the divine love for man.
Nor would the plurality of inhabited worlds endanger the dogma of the Redemption. Historically Jesus Christ immolated Himself on earth, it is objected; now if the other stars were also inhabited, how could those men have known the mystery and partaken of its effects? Here too the answer is simple: even granting that among the inhabitants of the other worlds there had also taken place a defection from obedience to God, as happened with Adam, how could one assert that the hand of almighty God had been 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" (Summ. Theol., 1a, q. 1, a. 2) also with respect to the earth.
Furthermore it is said: whoever reads the history 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 terrestrial 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 the man of the earth and to deny him the importance granted him by God. One must answer first of all that the revelation contained in Sacred Scripture does not have the purpose of informing us about cosmological and astronomical doctrines, and that therefore the inspired author adapts himself, in speaking, to the doctrines prevailing in his time. On the other hand, terrestrial man, even on the hypothesis of millions of other worlds, ennobled by the presence of man, would not at all lose the distinguished privilege granted him of having been raised to a sublime dignity by the fact that the Word became man on earth and assumed the nature of terrestrial man.
Finally it is objected that some ecclesiastical writers did not wish, even in past times, to admit the existence of the antipodes on earth; all the less would they be disposed to admit other men on the stars. The fact that some ecclesiastical writers have been been opposed to the existence of the antipodes is true; but the reason derived not from a consideration of habitability, but rather from the supposition that such men would not have been descendants of Adam. A false supposition, which had no real support in Catholic doctrine (v. ANTIPODI).
On the other hand, some would like to find in Sacred Scripture allusions favorable to the hypothesis of the habitability of the worlds, and they refer: 1) to the parable of the 99 sheep left by the good Shepherd to run after the one that was lost. Jesus would have left the inhabitants of the stars, supposed innocent, in order to come down to save the earthly inhabitants, who were sinners; 2) to the words of Jesus: "I have other sheep that are not of this fold... them also I must bring" (Jn 10:16). The "other sheep" would be the inhabitants of the stars, of whom Jesus Christ is also king; 3) to the phrase of St. Paul: "It pleased God... to reconcile all things in him... whether those on earth or those in heaven" (Col 1:20); since the good angels were certainly not redeemed by Jesus Christ, it would remain that 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!" (Hymn Lustra sex qui iam peregit, for Lauds in Passiontide); the redeeming blood would have regenerated not only the earth, that is, the earthly inhabitants, but also the heavens, that is, the celestial inhabitants.
To all these citations one must reply: these, and other similar texts, prove nothing as regards the question of the habitability of the worlds; their primary sense, according to the doctrine of the Church, is another; one may nevertheless say with certain theologians that, once the scientific demonstration of the plurality of inhabited worlds is established, such texts would also have alluded in veiled terms to the existence of these astral inhabitants.
In conclusion: on the day when science should succeed in proving that on other nearby or distant planets and on other stars there are also rational beings like us, philosophy will explain the origin of those men in the same way as for earthly men; that is, by recourse to the argument from causality, which postulates a creating Being. And theology will invite us to magnify still more the greatness, the goodness, the infinite prodigality of God.
Bibliography10 works
Celestino Testore