Can ‘invisible’ bodies be observed in the Solar System?

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.