一个新的类地行星形成模型（Hansen 2009，Walsh et al. 2011）探索了截短的原行星盘中的吸积，发现这样的配置能够重现太阳系中行星之间的质量分布，特别是地球/火星质量比，早期的模拟通常无法匹配。沃尔什等人测试了一种可能的机制来截断圆盘-木星和土星的两个阶段，向内然后向外迁移，正如在许多巨大行星形成的流体动力学模拟中发现的那样。除了截断圆盘和产生更真实的地球/火星质量比之外，巨行星的迁移还使小行星带充满了两种不同的天体-内带充满了起源于3天文单位以内的天体，外带充满了起源于巨行星之间和之外的天体（以下称为“原始”天体）。我们发现，在行星形成期间，行星将从分散到行星交叉轨道上的原始星子中吸积其总质量的1-2%。假设原始微行星中的水团质量分数为10%，该模型提供的水总量与今天地球上的水总量相当。虽然行星质量的径向分布和它们轨道的动力学激发与观察到的系统很好地匹配，但我们发现最后一次巨大的撞击通常早于2000万年，并且在该事件之后大量的质量被吸积。然而，在所有模拟中形成的27颗大于地球质量一半的行星中，有5颗确实经历了大的晚期撞击和随后的吸积，这与月球形成撞击的日期和该事件后地球吸积的估计质量一致。
英文摘要：
A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011) has explored accretion in a truncated protoplanetary disk, and found that such a configuration is able to reproduce the distribution of mass among the planets in the Solar System, especially the Earth/Mars mass ratio, which earlier simulations have generally not been able to match. Walsh et al. tested a possible mechanism to truncate the disk--a two-stage, inward-then-outward migration of Jupiter and Saturn, as found in numerous hydrodynamical simulations of giant planet formation. In addition to truncating the disk and producing a more realistic Earth/Mars mass ratio, the migration of the giant planets also populates the asteroid belt with two distinct populations of bodies--the inner belt is filled by bodies originating inside of 3 AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as `primitive' bodies).
We find here that the planets will accrete on order 1-2% of their total mass from primitive planetesimals scattered onto planet-crossing orbits during the formation of the planets. For an assumed value of 10% for the water mass fraction of the primitive planetesimals, this model delivers a total amount of water comparable to that estimated to be on the Earth today. While the radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system, we find that the last giant impact is typically earlier than 20 Myr, and a substantial amount of mass is accreted after that event. However, 5 of the 27 planets larger than half an Earth mass formed in all simulations do experience large late impacts and subsequent accretion consistent with the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event.
Two phase, inward-then-outward migration of Jupiter and Saturn in the gaseous Solar Nebula
最近的研究表明，如果巨行星经历了向内然后向外的迁移，类地行星和小行星带可以重现（“大钉”; Walsh et al 2011）。向内迁移发生在木星打开一个缺口时，II型向内迁移。当土星到达质量间隙并与木星发生3：2共振时，行星会向外移动。其目的是测试Grand Tack模型的可行性，并研究木星和土星从10个地球质量的核心成长过程中的动态演变。我们已经进行了数值模拟，使用基于网格的流体动力学代码。我们大多数的模拟假设一个等温状态方程的磁盘，但一个子集使用完全辐射版本的代码。对于一个等温盘，木星和土星的两相迁移是非常强大的，并独立于这些行星的质量增长的历史，如果磁盘是足够冷。对于辐射盘，我们发现一些结果与两相迁移和其他更复杂的行为。我们构建了一个简单的，1-D模型的不断发展的粘性磁盘计算磁盘的辐射特性的演变：磁盘从辐射到等温从其最外层区域向内的时间过渡。我们表明，一个两相迁移是一个自然的结果，即使在有限的假设下，等温条件是必需的。因此，我们的模拟提供了强有力的支持，大塔克的情况。
