SECOND BODY THEORIES. Laplace imagined his sun to be alone in space, even its nearest neighbours being too remote to influence it in any way. It was the natural supposition to make; we have already remarked how exceedingly rare an event it must be for two stars to approach near enough to influence one another. Yet no possible mode of evolution of a star which remains alone in space seems able to explain the origin of the solar system. As far back as 1750, Buffon had suggested that the solar system might have been produced through the disruption of the sun by another body, which he described as a “comet.” In propounding his Nebular Hypothesis, Laplace mentioned Buffon’s idea, but dismissed it somewhat curtly on the grounds that it seemed unable to account for the nearly circular orbits of the planets—an ill-founded objection, as we shall soon see. Yet when we find that a single star cannot of itself give birth to a solar system, it becomes natural to investigate what happens on the rare occasions on which the evolution of a star is directed along other paths by the near approach of a second star. In 1880, Bickerton of New Zealand, reviving Buffon’s idea, supposed that the solar system had been formed by the collision of the sun with another star. He imagined the débris of the collision to form a third nebulous body, condensations in which formed the planets. He shewed how the resistance which the planets would encounter as they moved through the surrounding nebula would gradually make their orbits more circular, and so account for their present nearly circular shapes. Ten years earlier, the English writer, R. A. Proctor, had advanced similar ideas, although with less precision. In 1905 Professors Chamberlin and Moulton of Chicago advanced a modification of the same ideas, under the name of the “Planetesimal Hypothesis.” Discarding the idea of material collision, they supposed that a passing star exerted a powerful tidal pull on the sun, with the result that the ordinary solar prominences temporarily attained an extraordinary violence; the ejected matter was supposed to rise to unusual heights and condense into small solid bodies, the “planetesimals,” out of the aggregation of which the planets were ultimately formed. These various theories were all purely speculative. They have shewn very little capacity either for surviving the acid test of mathematical analysis, or for explaining the more salient features of the solar system; none of them, for instance, explains why the larger planets in the solar system are accompanied by families of satellites.