If we suppose ourselves placed successively on Mars, Jupiter, Saturn, Uranus, and Neptune, the Sun will successively appear smaller and smaller, and at Neptune it will still have a round and distinctly defined disc. At greater distances beyond our system the disc of the Sun would be seen only through a telescope, and all the planets, except Jupiter and Saturn, will have disappeared. At a greater distance still they will vanish in succession, and before we cross the immense void which lies between our system and the nearest system of the stars, our Sun will be seen as a single star twinkling in the sky. All his planets, primary and secondary, and all his comets, will have disappeared in the distance.

Hence we are led to believe that the fixed stars are the suns of other systems, whose planets are invisible from their distance. As no change of place has been observed in single fixed stars, excepting that which is common to them all, and arises from the motion of our system, we are entitled to consider these single stars as the centres of systems like our own; to suppose them without planets, and to be merely globes of light and heat, would be contrary to analogy as well as to reason. We know that there is one star in the universe surrounded by planets, and one of these planets inhabited; and when we see another single star equal, if not greater in brilliancy, we are entitled to regard it as the centre of a system, and that system with at least one inhabited planet.