The Formation of the Giant Planet Cores

木星和土星是我们在理解行星形成过程中面临的最具挑战性的问题之一。这些主要由氢和氦组成的行星，在太阳星云消散之前一定已经吸积了这种气体。对年轻恒星系统的观察表明，气盘的寿命为100万到1000万年。所以，气态巨行星一定是在这个时间之前形成的。木星和土星形成的主要理论是所谓的核心吸积模型。在这个模型中，首先形成了一个大的行星胚胎，主要是通过两体吸积形成的。然后是一段时期的星云气体直接流入这颗不断增长的行星。大质量大气的吸积需要一个质量约为10个地球质量的固体核心。事实证明，在星云消失之前组装这么大的天体，对目前的行星形成理论提出了一些严重的挑战。在过去五年左右的时间里，行星形成界为克服这些问题作出了协调一致的努力。事实上，在文献中提出了几个新的想法。然而，这些想法中的许多还没有通过现代的动态模拟得到充分的探索。哈罗德·莱维森博士和他的同事将使用最先进的N体数值方法构建迄今为止最全面的岩心吸积模型。特别是，他们将进行一系列模拟，研究嵌入在太阳星云中的行星胚胎和行星体系统的演化。从长远来看，这些模型将包括：i)行星体和胚胎的直接引力相互作用；ii)行星体气动力阻力；iii)胚胎的第一类迁移；iv)碎裂；v)胚胎大气的影响；vi)所谓的雪线上的固体堆积；以及vii)湍流驱动的胚胎迁移。它们将从相对简单的计算开始，并随着研究计划的进展而增加复杂性。这种详尽的模型应该可以解决木星和土星形成的关键问题，或者证明需要更有创造性的解决方案。解决木星和土星的形成问题将对广泛学科的研究人员具有重要的科学意义，原因有两个。首先，对于那些对太阳系感兴趣的人来说，木星和土星在较小的程度上控制了整个系统的动态演化。因此，这项研究涉及小行星带的演化、奥尔特云的形成、向地球输送水、天王星和海王星的形成以及柯伊伯带的雕刻等问题。此外，木星和土星很可能与迄今发现的大约200颗太阳系外行星相似。因此，了解我们这两颗本土气体巨星的形成，将深刻影响我们对它们远亲的理解。在这项研究的过程中，研究小组将通过会议报告和在被引用的期刊上发表文章来公开传播结果。莱维森博士把他所有的演示文稿都放在了网上。专业天文学家、教育工作者和新闻媒体利用这一资源作为太阳系起源的信息来源，以及图形和动画的来源。他经常参加教育和公共宣传项目，包括在电视和电台露面。***

AST 0708775LevisonJupiter 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 accretionmodel. 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 ofgraphics and animations. He frequently participates in educational and public outreach projects,included television and radio appearances. ***