Description:
<jats:p>The idea that the Solar System possesses a primordial isotropic cloud of comets has been with us in quantitative form for about thirty years (Oort 1950). A considerable edifice has been built on this proposition (e.g. Weissmann 1982), which has proved durable despite indications that the cloud may in fact be significantly non-thermal and display Galactic alignments (Tyror 1957, Richter 1963, Oja 1975, Yabushita <jats:underline>et al</jats:underline>. 1979, Radzievsky 1981). However with the discovery in recent years of a system of massive molecular clouds in the Galactic disc, it has become apparent that the environment in which the Oort cloud has to survive is very different from that envisaged in 1950; a re-appraisal of the standard picture is therefore called for.</jats:p><jats:p>One corollary of the discovery of the molecular clouds is that the traditional objections to an interstellar comet cosmogony, such as the lack of observed hyperbolic comets and the difficulty of growing and capturing them, may no longer apply (see for example the recent review by Clube & Napier 1982a, hereinafter CN). Another consequence, reviewed here, is that tidal effects due to molecular clouds may be so large that the Oort cloud is frequently disrupted (CN; Napier & Staniucha 1982). In this situation frequent replenishment is implied and it is therefore necessary to discriminate between the <jats:underline>primordial</jats:underline> and the <jats:underline>observed</jats:underline> Oort cloud and to consider possible alternative sources for the latter. Such an enquiry would provide constraints on the as yet unsolved problem of comet formation.</jats:p>