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Astron. Astrophys. 361, 369-378 (2000)

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1. Introduction

The origin of the Oort cloud comets still remains an unanswered question, because all recent theories had serious shortcomings.

The most popular primordial theory at present, which assumes the creation of comets in the Uranus-Neptune region during planet formation, does not provide a sufficient source of energy to eject the cometary nuclei into the Oort cloud region. The initial Oort cloud cometary population exceeded at least 80 [FORMULA] (Earth masses) (Weissman 1990). Bailey (1994) estimated that the present cometary mass must be at least 380 [FORMULA], for which the upper limit of survival probability is 20[FORMULA]. That means the ejected mass of cometary population was several, if not many, times higher than the sum of masses of Uranus and Neptune, which were regarded as the main ejecting planets within the original concept.

Fernández (1997) tried to solve the problem of the energy needed to deliver the cometary nuclei into the Oort cloud; he assumed that a considerable fraction of the nuclei was ejected into the cloud not only by Uranus and Neptune, but by Jupiter and Saturn as well. In the classic scenario, the efficiency of the latter planets to place the nuclei into the cloud was too small, because they were very likely to overshoot the narrow energy range and eject the nuclei into interstellar space (the ratios of mass ejected to the comet cloud and interstellar space are only 0.03 for Jupiter and 0.16 for Saturn (Fernández 1985)). To remove this controversy, Fernández (1997) suggested an improved scenario in which the solar system forms within a dense galactic environment, perhaps a molecular cloud and/or an open cluster, which produces a more tightly bound comet reservoir. Due to their different birthplaces, the comets should exhibit a different physical-chemical nature. On this point, the new scenario still lacks observations in its favour, because some evidence has been found indicating that cometary nuclei were created in a cooler environment than the Jupiter-Saturn zone of protoplanetary dics. This was most recently documented by Bergin et al. (1999), who demonstrated that comets Halley and Hyakutake had to form at temperature 25 to 30 K, comet Hale-Bopp at 40 K.

The theories of interstellar origin cannot provide any suitable mechanism of capture of interstellar comets by the solar system (Valtonen & Innanen 1982; Torbett 1986). Moreover, these have been disregarded because of the density argument (e.g. Rickman & Huebner 1990).

Perhaps the only theory without any shortcoming is the theory of creation in situ published by Hills in 1982. He suggested that pressure due to the radiation from the Sun and neighbouring protostars may have forced the coagulation into comets of dust grains in collapsing layers of the protosun at distances from 1 to 5 thousand AU (astronomical units). However, if this mechanism is actually efficient in comet creation, then these would also have been coagulated, at an even higher rate, in molecular interstellar clouds during the passages of highly luminous stars through the dense cloud regions.

We know that there are many indications of similarities among cometary material and that of cool, dense interstellar clouds (e.g. Greenberg 1982, 1998; Clube & Napier 1985; Mumma et al. 1990; Delsemme 1991; Greenberg & Shalabiea 1994; Greenberg & Li 1998; Bergin et al. 1999). On the other hand, it can be regarded as proven that the comets have been bounded to the solar system over all its existence (all observed comets have been bounded to this system (Marsden et al. 1973)). Combining these facts and considering the previously mentioned shortcomings, it seems that molecular interstellar clouds are the most appropriate birth-place of comets. If this assumption is accepted, then the cometary nuclei also had to be present in the protosolar nebula before its collapse into the protosun and protoplanetary disc.

In this paper, we attempt to answer the question of whether the cometary nuclei remained at Oort cloud distances after the protosolar nebula collapse, in a more exact and complete way than in our previous studies (Neslusan 1994, 1999). We know the cometary nuclei had to take part in the collapse in a different way than the gaseous and dusty components of the nebula. The atoms and molecules lost their kinetic energy by the well-known mechanism suggested by Hayashi (see his review from 1966, e.g.), but this mechanism could not be efficient with such large bodies as cometary nuclei. If the nuclei were present in a cloud during its collapse, then these did move mechanically whilst the atoms and molecules followed the laws of hydrodynamics. Of course, the trajectories of the cometary nuclei would have been changed due to the change of gravitational potential, but that does not obviously imply their collapse into the protoplanetary disc. In other words, we suppose that the comets of our Oort cloud could represent a remnant of protosolar nebula from a stage before its collapse into the protosun and protoplanetary disc.

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© European Southern Observatory (ESO) 2000

Online publication: September 5, 2000