It is, therefore, upon the question of the presence of water that the question of the habitability of a given world chiefly turns.
The warmth and the habitability of the earth's surface is due to the presence of water-vapor and carbon-dioxide in the atmosphere.
Thus is revived the old controversy between Whewell and Brewster as to the habitability of the planets.
The first circumstance to be considered in relation to the habitability of a planet is its distance from the sun.
But its case shows that a further condition of habitability has to be satisfied by a planet.
Neither of these facts was known when Proctor wrote upon the question of the habitability of the planets.
But without regard to the question of habitability, the asteroids will be found extremely interesting.
We learn, therefore, that the time of rotation of a planet is an important factor in its habitability.
When we look to the other planets of our system we see everywhere illustrations of the relation of size and mass to habitability.
It was the discovery that the moon has no perceptible atmosphere that first seriously undermined the theory of its habitability.