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Published online by Cambridge University Press: 14 August 2015
Although the problem of the origin of comets has not yet been settled, there are only two possibilities; one is to regard that comets formed within the solar system when or soon after planets formed, and the second possibility is that comets have been captured from interstellar space by planetary perturbations or by perturbations of the galactic nucleus. To decide upon the possibility, one has to take into account statistical features of cometary orbits.