Forming Planetesimals in Solar and Extrasolar Nebulae
Forming Planetesimals in Solar and Extrasolar Nebulae
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DOI：
10.1146/annurev-earth-040809-152513
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发表时间：
2009-09
影响因子：
14.9
通讯作者：
E. Chiang;A. Youdin
中科院分区：
文献类型：
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作者：
E. Chiang;A. Youdin
行星是由星子构成的：大于一公里的固体，通过成对碰撞而形成。星子本身不太可能仅由两体碰撞形成；亚千米级物体的引力场太弱，静电引力也太弱，无法增长到几厘米以上。我们回顾了自引力将大量小粒子聚集在一起时形成星子的可能性。即使在自重力较弱的情况下，空气动力学过程也会积累相对于气体的固体，为引力坍缩铺平道路。粒子沿径向向内漂移时堆积起来。气体湍流搅动粒子，但也会使它们聚集在一起，从而播下坍缩的种子。虽然固体对气体的反馈会引发垂直剪切不稳定性，从而阻碍自重力，但同样的反馈会引发流不稳定性，从而强烈集中颗粒。数值模拟发现，10-100厘米大小的固体在紊流盘中的引力坍缩。我们概述了需要取得进展的领域，包括更小的物体自引力的可能性。
Planets are built from planetesimals: solids larger than a kilometer that grow by colliding in pairs. Planetesimals themselves are unlikely to form by twobody collisions alone; subkilometer objects have gravitational fields individually too weak, and electrostatic attraction is too feeble for growth beyond a few centimeters. We review the possibility that planetesimals form when self-gravity brings together vast ensembles of small particles. Even when self-gravity is weak, aerodynamic processes can accumulate solids relative to gas, paving the way for gravitational collapse. Particles pile up as they drift radially inward. Gas turbulence stirs particles but can also seed collapse by clumping them. Whereas the feedback of solids on gas triggers vertical-shear instabilities that obstruct self-gravity, this same feedback triggers streaming instabilities that strongly concentrate particles. Numerical simulations find that solids ∼10–100 cm in size gravitationally collapse in turbulent disks. We outline areas for progress, including the possibility that still smaller objects self-gravitate.