Since Safronov's introduction of his planetesimal accretion model for terrestrial planets and Mizuno's extension to the formation of Jupiter and Saturn, theoreticians have made substantial progress in understanding planet formation in the last few decades. At the time of this conference, the CAGC simulations have provided the following conclusions that can be summarized as follows (Fig. 3):
(1) The opacity due to grains in the protoplanetary envelope has a major effect on the formation timescale, but no effect on the core mass. It is not possible for a gas giant with a small solid core to form on a short timescale for models computed with the grain opacity equal to that of typical interstellar material. For models computed with the grain opacity below interstellar values, formation times are short, but the final core mass is unaffected. The baseline case computed in HBL05 (10LTO) shows that Jupiter can be formed at 5 AU in just over 2 Myr, but the core mass is 16 M®.
(2) Halting the planetesimal accretion provides for formation times to be in the range of 1-4.5 Myr and for a core mass consistent with that of Jupiter, if the initial solid surface density in the disk is three times that of the minimum mass solar nebula.
(3) By reducing the initial solid surface density in the disk to two times that of the minimum-mass solar nebula, it is still possible to form Jupiter in less than 5 Myr if the core accretion is cutoff at 5 M® .
(4) All models satisfy the constraint that the total heavy element abundance is less than, or comparable to, the value deduced from observations of Jupiter. A few models have low heavy element abundance (3-5 M®), but it is quite reasonable to expect the planet to accrete more solids during or after rapid gas accretion, which is not taken into account in these models.
(5) The results of Paper 1 show that Phase 2, the early gas accretion phase before crossover mass is reached, essentially determines the timescale for the formation of a giant planet. However, the results presented in HBL05 indicate that for low atmospheric opacity and/or a cutoff in accretion of solids, Phase 2 can be relatively short. Thus, the time for Phase 1, the solid core accretion phase, may be the determining factor.
(6) Migration, which prevents the depletion of the feeding zone that occurs in in situ formation, appears to have a very important effect on the formation timescale by decreasing it by about a factor of 10 (Alibert et al. 2005), without having to consider massive disks (Lissauer 1987).
Although some old problems relating to planet formation have been resolved, there are others that still need investigating. Taking into account multiple embryos and a number of other physical processes, recent simulations of the core accretion process indicate that the core formation times are longer than those computed by the ARC/UCSC studies (Inaba et al. 2003; Thommes et al. 2003; Kokubo & Ida 2002). Thus, the question remains as to how large an enhancement of solid surface density, as compared to that in the minimum mass solar nebula, is needed to form a giant planet in a few Myr. Another problem is related to the migration of a gas giant planet. According to Type I migration calculations by Tanaka et al. (2002), it seems difficult to form a planet and prevent it from spiraling g 10
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