Pagina:Scientia - Vol. VII.djvu/254

246 scientia

Meteors are such complex bodies both in structure and composition that it is difficult to conceive of them as the primitive form of matter; they are more like the debris of earlier worlds. This idea naturally leads us on to the planetesimal hypothesis, which has been lately put forward by Professors Moulton and Chamberlin. According to this view, the sun existed in past ages in solitary state, without any attendant worlds; at some very remote epoch, another sun is supposed to have passed extremely near it, without actually colliding, but causing immense tidal disturbance, by which an appreciable fraction of its total mass was torn off; 1/700 of its mass went into the known planets, a much larger amount returned to the sun, but some remained unattached, and formed our comets and meteor swarms. It was the perturbing action of the other sun that gave the ejected mass moment of momentum, and thus prevented it from falling back on the sun. If we suppose that before the cataclysm our sun had already cooled sufficiently for a solid crust to form, which would absorb a portion of the hydrogen and other gases in the atmosphere, we seem to get an explanation of the large solid meteoric masses that frequently fall on the earth and which contain a large quantity of occluded gases. The theory of course implies that the sun’s temperature was again raised as a result of the appulse, either by actual collision, or the impact of the return of part of the ejected matter.

The chief drawback to the adoption of the planetesimal hypothesis seems to me to be the extreme improbability of such a near approach of two suns to each other. The interstellar distances are so immense compared with the size of each sun that such encounters must be excessively rare. The frequent appearance of Novae is sometimes quoted as evidence in favour of such appulses; these outbursts occur, almost without exception, within the Galaxy, where we have good reasons for supposing the star-density to be much greater than elsewhere, moreover the very rapid decline in the light of Novae suggests that they are not stars in an advanced stage of condensation, but are in a much more diffused and tenueus state. On account of these difficulties I think that we should only regard the planetesimal hypothesis as a plausible conjecture, not as an establised conclusion.