ABITABILITÀ DEI MONDI. —
1. ACCORDING TO SCIENCE
By the habitability of worlds is meant a set of conditions suitable for the forms of life existing on Earth, without denying the absolute possibility of forms of life suited to biospheres totally different from Earth’s.A first examination concerns our immediate surroundings: the solar system. It may be said that astronomers are generally agreed that the planets, with their satellites, in their present conditions are not habitable by beings organized like terrestrial ones. Mercury has no atmosphere, and, always presenting the same face to the Sun, the face at the center
has a temperature of about 350° C., while on the opposite face perpetual night reigns, with cold reaching almost absolute zero. Venus is covered with dense clouds of carbonic acid, while the spectroscope has shown the absence of water vapor and oxygen. Since its rotation is very probably extremely slow, the solar rays penetrating as far as the surface cause a temperature there at the center comparable to 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 ground suitable for life, and the lack of oxygen and the abundance of carbonic acid confirm this view. On Mars there are conditions 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 the winter nights it falls to —90° C. But the atmosphere, which is extremely tenuous, contains only a very small quantity of oxygen and carbonic acid, and perhaps not even that; vegetation comparable to terrestrial vegetation is therefore impossible there, though there may be some moss-like vegetation, of which the change from green to brown observed in certain dark patches as the Martian season varies, toward the summer solstice, would be an indication. It nevertheless seems that the spectrum of the dark patches lacks the reflection characteristic of terrestrial vegetation, namely chlorophyll. In short, Mars too offers no ground suitable for higher forms of life. The Moon has neither air nor water, and its temperature varies between 120° C. and —80° C. On Jupiter, with its mephitic atmosphere of methane and ammonia, without oxygen, carbonic acid, or moisture, and with a temperature of —130° C., every form of life is entirely ruled out. The same applies to Saturn, Uranus, Neptune, and 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 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 celestial space outside the solar system, we have no information. The stars, even the nearest, are so far distant from us that any planets, even those the size of Jupiter, lie beyond the reach of the most powerful telescopes.
Hence the habitability of worlds outside the solar system is a matter of pure fantasy.