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Published online by Cambridge University Press: 14 August 2015
The different hypotheses proposed so far on the origin of comets have been reviewed in detail last year (Delsemme 1977). In particular, I mentioned why the orbital statistics seem to be consistent with only one hypothesis, namely that comets were accreted at unknown but moderate distances (10 to 1000 A.U.) within the protosolar nebula, and ejected later into a sphere whose radius is some 50,000 A.U., usually called the Oort's cloud. Safronov (1977) and Cameron (1977) have different scenarios to do just that. Even with improvements like that of Dermott and Gold (1978) the choice between these scenarios will remain impossible until we have a convincing model describing the gravitational collapse of an interstellar cloud into a planetary system.