The number of stars revealed by a telescope of moderate power amounts to 10 millions. If we suppose that every star only has on an average one planet, we obtain already the considerable number of 10 million planets. It is true that with us three planets out of eight may be considered habitable, but let us suppose the proportion in the universe is only one in a hundred. We shall still have no less than a hundred thousand habitable planets.
This number, evidently falling short of reality, makes already quite another figure compared with the three habitable worlds we had before.
After that the universe no longer appears such a desert. There is nothing to hinder us from giving rein to our fancy and imagining on the one hand the strangest forms of life among the innumerable planets gravitating round the stars, and on the other hand thinking beings which surpass us considerably in intelligence and to whom our most difficult problems are as transparent as self-evident truths. The conclusion is that in all sorts of degrees we must see Life radiating into space and lighting up Infinity.
REMARKS
To this interesting study of Monsieur Scheiner we may add in the first instance that the question is still more complex, that we can consider the resources of Nature as infinite, and that “positive” science founded only upon our senses is quite insufficient, though it may be the only basis of our reasoning. It is through the eyes of the spirit that we must survey the universe.
As we have seen, this new examination of the question of the habitability of other worlds by thinking beings presents a new interest, and the author has been able to escape the usual error of scientific writers, which consists in supposing that the first condition of habitability of another world is that it resembles the Earth. That is always the reasoning of the fish, which would affirm without an absolutely logical conviction irrefutable to himself that it is impossible to live outside water. But it seems to us that our conception of the universe can be even more vast and elevated than that of the learned German astronomer.
As regards the planetary systems differing from ours, we are no longer obliged to fall back upon suppositions. We know already with certainty that our Sun is no exception, as some theoreticians maintained even quite recently. The discovery is of considerable interest.
It is surely a rather exceptional situation that a sidereal system consisting of a central sun and one or more bodies gravitating round it should have its system just in our line of vision so that the revolution of the bodies which compose it should bring dark bodies between us and the star and produce a more or less complete eclipse. Since, on the other hand, such eclipses would be our only means of proving the existence of these unknown planets (except perturbations as in the case of Sirius and Procyon), it seems that it would have been absolutely daring to hope for such a circumstance to discover solar systems different from ours. Yet this exceptional case occurs in various parts of the sky. Thus, for example, the variable star Algol owes its variation of brightness, which reduces it from the second to the fourth magnitude at intervals of 69 hours, to the interposition of a body between it and the Earth, and celestial mechanics has already been able to determine with precision the orbit of this body, its dimensions and its mass, and even the ellipticity of the Algol sun. Thus we have here a system of which we know the sun and one enormous planet whose revolution takes place in 69 hours with a very high velocity as measured in the spectroscope, a planet still self-luminous, although less luminous than its sun, as the Earth was long ago, but a planet in the course of cooling and approximating to the state of Jupiter.
The star Delta Cephei is in the same case: it is an eclipsing variable with a period of 129 hours, and its eclipsing planet also revolves in the plane containing our line of vision. The star U Ophinchi shows a similar system, and observation has revealed several others.
If therefore chance has brought it about that a certain number of different solar systems should have been revealed to terrestrial observation by presenting side views, that is evidence of the existence of innumerable solar systems disseminated through the depths of space, and we are no longer reduced to mere conjectures.