The observational astronomer generally feels only an indirect interest in the problem of how our earth and its companion planets came into being; his telescope can give him no direct information on the subject, since such planets as other suns may possess are too small and too distant to be observed. If every star in the sky were suddenly to give birth to planets we should in all probability remain unaware that anything was happening.
Yet the problem is of thrilling interest to science in its widest sense. The old nebular hypothesis of Laplace had pictured the stars as shrinking nebulae, which rotated faster and faster as they shrank, and threw rings of matter off their equators in the process, each of which was destined in time to condense into a planet. This cosmogony implied that the shedding of planets was a normal event in the life of a star. It led to the concept, so commonly held in the nineteenth century, that every star in the sky was a sun distributing light and heat to a retinue of worlds circling round it. As solar light and heat are the most obvious essentials for terrestrial life, it was natural to take the next step and assume that every star we saw in our telescopes was busily at work radiating energy to maintain life on its surrounding planets.