An interstellar origin for comets

It is usually supposed that the outer regions of the Oort cloud have been thermalised by the cumulative effect of stellar encounters and that the long-period comets are perturbed into the inner solar system by passing stars. This heating, or relaxation, is a gentle process. However it is a well known result in stellar dynamics that the relaxation of stellar motions in the Galaxy is almost entirely due to the presence of large masses, such as nebulae or star groups, individual stellar encounters taking approximately 104-105 times as long to produce the same degree of relaxation. It is therefore necessary to consider the effect of large perturbing masses on a primordial cloud of comets. The author finds that the outer regions of such a cloud (say beyond 2a approximately 20,000 AU) would probably have boiled off by now. (Auth.)