Abstract
MANY theories of the origin of comets predict that there are ∼ 1011 comets in the Solar System; most of them, unfortunately, at such a distance that they can never be observed. Some theories go even further and predict that, in addition to these comets in the Oort cloud, the remains of a primaeval comet belt may still exist at a distance of ∼ 50 AU. This idea has been investigated by Hamid et al.1, who show (from an analysis of observed perturbations on Halley's comet) that the total mass of the comet belt must be ≲ 1 Earth mass. Even this relatively small mass would be enough for ∼ 1010 comets—a significant fraction of the total of the whole Oort cloud. It has been noted that comets contain a relatively large fraction of‘heavy’elements, and it has been suggested2 that the total galactic population of comets may be large enough to affect significantly the chemical evolution of the Galaxy by acting as a ‘sink’ for the heavy elements.
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