Dynamics of Extrasolar Planets and the Kuiper Belt

系外行星和海王星外天体的埃奇沃斯-柯伊伯带构成了行星天文学的两个强大驱动力。自从发现这些天体不到十年前以来，对它们动力学特性的理论理解仍然难以捉摸。为什么太阳系外行星的轨道偏心率比太阳系内的气态巨行星大得多？是什么在47天文单位的日心距离上描绘出经典柯伊伯带的明显边缘？经典带天体的轨道倾角是如何急剧膨胀的？加州大学伯克利分校的Eugene Chiang博士及其同事将研究一系列理论动力学问题，直接解决这些问题。他将研究这两种看似不同的物体如何参与许多相同的动力学过程，最显著的是轨道迁移和共振相互作用。行星在其出生的气态盘中时可以非常灵活地移动。已知的太阳系外行星质量足够大，它们可以在轨道周围的圆盘物质中清除环状空隙。开隙行星受制于其宿主盘的粘性演化。磁流体动力学湍流盘的粘性扩散时间随着离恒星距离的减小而缩短。因此，两颗打开间隙的行星向其母恒星迁移，使得外行星与内行星的周期之比增大。这个比率的发散意味着一系列的平均运动共振将会交叉。每次共振交叉都会在迁移体中产生大量的轨道偏心率。蒋博士和他的合作者将探索这种令人兴奋的古怪现象的机制。将计算原行星盘的粘性、热量和质量分布，以确定行星迁移的时间尺度。共振交叉的天体力学将通过一系列的解析和数值轨道积分进行研究。将进行行星-圆盘相互作用的数值流体动力学模拟，以测试共振通道的有效性。太阳系巨行星轨道的圆度可以与系外行星轨道在原行星盘框架内的极端伸长相协调，原行星盘的粘度随着距离的增加而急剧下降；这个框架表明，外太阳系的轨道结构可能确实很普遍。海王星最外层，最强的2:1平均运动共振对经典柯伊伯带的引力雕刻程度将被确定。当海王星在后期重轰击时期向外迁移时，它的共振向外扫去，并将柯伊伯带天体捕获到振动轨道上。蒋博士和他的合作者将通过解析和数值计算来解释在2:1共振范围内振动的物体的有限质量。一个巨大的、迁移的2:1共振可能会将物体泄漏到经典带域；共振捕获和保持的不完美效率决定了2:1中存在的体的比例和组成非共振经典带的体的比例。将测量由偏心、倾斜2:1摄动器对经典带天体的动态加热程度。这项研究最终将在2:1共振范围内预测质量，蒋博士已经积极参与了专门的观察，可以对其进行测试

AST 0205892ChiangExtrasolar planets and the Edgeworth-Kuiper Belt of trans-Neptunian bodies constitute twopowerful drivers of planetary astronomy. Since the discovery of these objects less than a decadeago, theoretical understanding of their dynamical characteristics remains elusive. Why are theorbital eccentricities of extrasolar planets so large compared to those of Solar System gas giants?What is responsible for delineating an apparent edge to the Classical Kuiper Belt at a heliocentricdistance of 47 AU? How did the orbital inclinations of Classical Belt objects become dramaticallyinflated? Dr. Eugene Chiang and colleagues, at the University of California at Berkeley, will investigate a series of theoretical dynamical problems that directly address these issues. He will examine how these two seemingly disparate classes of objects participate in many of the same dynamical processes, most notably orbital migration and resonant interaction. Planets can be remarkably mobile while embedded in their natal gaseous disks. Known extrasolar planets are sufficiently massive that they clear annular gaps in disk material about their orbits. A gap-opening planet is slaved to the viscous evolution of its host disk. Viscous diffusion times of magnetohydrodynamically turbulent disks shorten with decreasing distance from the star. Thus, two gap-opening planets migrate towards their parent star such that the ratio of the period of the outer planet to that of the inner planet grows. The divergence of this ratio implies that a series of mean motion resonances will be crossed. Each resonance crossing can generate substantial orbital eccentricities in the migrating bodies. Dr. Chiang and his collaborators will explore this mechanism for exciting eccentricities. The viscous, thermal, and mass profiles of protoplanetary disks will be computed to determine planetary migration timescales. The celestial mechanics of resonance crossings will be investigated through a series of analytic and numerical orbit integrations. Numerical hydrodynamic simulations of planet-disk interactions will be undertaken to test the effectiveness of resonance passages. The circularity of orbits of Solar System giants may be reconciled with the extreme elongations of extrasolar planetary orbits within the framework of protoplanetary disks whose viscosities decrease dramatically with distance; this framework indicates that the orbital architecture of the outer Solar System may indeed be commonplace.The extent to which Neptune's outermost, strongest 2:1 mean-motion resonance gravitationally sculpted the Classical Kuiper Belt will be ascertained. As Neptune migrated outwards duringthe era of late heavy bombardment, its resonances swept outward and captured Kuiper Belt Ob-jects into librating orbits. Dr. Chiang and his collaborators will account for, through analytic andnumerical calculations, the finite masses of bodies librating within the 2:1 resonance. A massive,migrating 2:1 resonance may leak objects into the Classical Belt domain; imperfect efficiencies ofresonant capture and of retainment determine the fraction of bodies that reside in the 2:1 and thefraction of bodies that comprise the non-resonant Classical Belt. The degree of dynamical heatingby eccentric, inclined 2:1 perturbers on Classical Belt Objects will be gauged. This study willculminate in predictions for the mass within the 2:1 resonance that can be tested by dedicatedobservations in which Dr. Chiang is already actively involved.***