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L. Dones, H. F. Levison (SwRI), M. J. Duncan (Queen's University), P. R. Weissman (JPL)
The standard model for the formation of the Oort cloud (OC) of comets assumes that the giant planets pump small bodies into orbits with large aphelion distances. Galactic tides and passing stars then raise the bodies' perihelia out of the planetary region. Dynamical simulations have shown that only ~2.5% of the planetesimals with initial semi-major axes, a, between 4 and 40~AU should reside in the outer Oort cloud (a > 15,000~AU) at present ([1]). Assuming an outer OC population of 1012 comets and an average cometary mass of 4 \times 1016~g ([2]), the original mass in planetesimals between 4 and 40~AU was {~250M\oplus, some 4 times the mass in solids in a minimum-mass solar nebula. This amount of mass likely would have produced excessive migration of the giant planets ([3]). Furthermore, the predicted population of the inner OC is comparable to that of the outer OC ([1]), whereas the observed orbital distribution of Halley-type comets ([4]) appears to require a massive inner OC as a source region. Several groups have recently considered other physical effects that may help to resolve these discrepancies ([5]-[7]). We will review these effects, and will present new calculations which include (i) gas drag in the solar nebula, (ii) the growth of Jupiter and Saturn, and (iii) small body eccentricities excited prior to the formation of the OC. Finally, we will discuss whether these effects can resolve the discrepancies between the standard model and observations. This work was supported by grants from the NASA PG&G (LD,PRW) and Origins (HFL) programs.
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[1] Dones, L., et al. (2000). Bull. Amer. Astron. Soc. 32, 1060. [2] Weissman, P. (1996). In Completing the Inventory of the Solar System (T.W. Rettig and J.M. Hahn, Eds.), pp. 265--288. [3] Hahn, J.M., and Malhotra, R. (1999). Astron.~J.~117, 3041. [4] Levison, H.F., et al., (2001). Astron.~J., April 2001. [5] Fernández, J.A., and Brunini, A. (2000). Icarus 145, 580. [6] Stern, S.A., & Weissman, P.R. (2001). Nature 409, 589. [7] Leitch, E.M. & Vasisht, G. (1997). New Astronomy 3, 51.