Jupiter and Saturn represent one of the most challenging problems we face in our understanding of planet formation. These planets, which are made mainly of hydrogen and helium, must have accreted this gas before the solar nebula dispersed. Observations of young star systems suggest that gas disks have lifetimes of 1 to 10 million years. So, the gas giant planets had to have formed before this time. The leading theory for the formation of Jupiter and Saturn is the so-called core accretion model. In this model a large planetary embryo formed first, mainly by two-body accretion. This is then followed by a period of inflow of nebular gas directly onto the growing planet. The accretion of a massive atmosphere requires a solid core of ~10 Earth mass. Assembling such a large body before the nebula disappears, it turns out, offers some serious challenges to the theory of planet formation as it currently stands. In the last five years or so, there has been a concerted effort by the planet formation community to overcome these problems. Indeed, several new ideas have been presented in the literature. However, many of these ideas have yet to be fully explored with modern dynamical simulations.
Dr. Harold Levison and colleagues will construct the most comprehensive model of core accretion to date using state-of-the-art N-body numerical methods. In particular, they will perform a series of simulations that study the evolution of a system of planetary embryos and planetesimals embedded in the solar nebula. In the long run, the models will include: i) the direct gravitational interaction of the planetesimals and embryos, ii) aerodynamic drag on the planetesimals, iii) Type I migration of the embryos, iv) fragmentation, v) the effects of embryos' atmospheres, vi) the buildup of solids at the so-called snow-line, and vii) turbulence-driven migration of the embryos. They will begin with relatively simple calculations and increase the complexity as the research program progresses. Such exhaustive modeling should allow solution of the critical problem of the formation of Jupiter and Saturn, or prove that a more creative solution is needed.
Solving the problem of the formation of Jupiter and Saturn will have important scientific implications for researchers in a broad range of disciplines for two reasons. First, for those interested in the Solar System, Jupiter, and to a lesser extent Saturn, has controlled the dynamical evolution of the entire system. Thus, this research is relevant to such issues as the evolution of the asteroid belt, the formation of the Oort cloud, delivery of water to the Earth, the formation of Uranus and Neptune, and the sculpting of the Kuiper belt. In addition, Jupiter and Saturn are probably similar to the roughly 200 extrasolar planets thus far discovered. Therefore, understanding the formation of our two home-grown gas giants will profoundly affect our understanding of their distant cousins. During the course of this research, the team will openly disseminate the results through conference presentations and publications in refereed journals. Dr. Levison puts all his presentations on the Web. This resource has been used by professional astronomers, educators, and the news media as a source of information on the origin of the Solar System, as well as, a source of graphics and animations. He frequently participates in educational and public outreach projects, included television and radio appearances. ***