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链式共振构型多行星系统的形成

The Formation of Multi-planetary Systems in Chain Resonance

  • 摘要: 链式共振构型是系外行星系统中的常见构型. 研究链式共振构型多行星系统的形成与演化可以为系外行星的形成研究提供新的线索. 通过筛选得到周期比符合链式共振构型的多行星系统, 并统计其相邻两行星周期比分布特征, 发现相邻两行星处于3:2平运动共振(Mean Motion Resonance, MMR)附近更多, 2:1 MMR附近更少, 表明链式共振构型多行星系统轨道构型更紧致. 通过数值模拟方法研究链式共振构型的形成与演化, 构建稳定进入2:1 MMR的两行星系统作为参照组. 模拟结果表明若参照组轨道最外侧存在第3颗行星, 迁移过程中最外侧行星可以使内侧行星对由2:1 MMR迁移至3:2 MMR, 且最外侧行星质量增加使内侧两行星和外侧两行星进入3:2 MMR的概率增加, 最外侧行星半长轴减小可以使内侧行星进入3:2 MMR的概率增加. 若轨道最外侧存在第4颗行星, 行星进入共振的时间顺序可能是系统构型不同的主要因素. 当最外侧两行星先于内侧行星进入共振, 最外侧两行星容易进入3:2 MMR, 且最外侧两行星向内迁移可以使内侧两对行星进入3:2 MMR, 形成更紧致的轨道构型. 模拟结果验证了链式共振构型系统观测数据的相邻两行星周期比分布, 为链式共振构型多行星系统呈现更紧致轨道构型的观测特征提供了解释.

     

    Abstract: Chain resonance configuration is a common architecture in exoplanetary systems. Investigating the formation and evolution of multi-planet systems with chain resonances can provide new clues to the understanding of exoplanet formation. By selecting multi-planet systems whose period ratios match chain resonance configurations and analyzing the distribution of period ratios between adjacent planet pairs, we find that planet pairs are more frequently located near the 3:2 mean motion resonance (MMR) and less near the 2:1 MMR, indicating that chain resonance systems tend to have more compact orbital architectures. Numerical simulations are used to study the formation of such resonant chains. A basic model with two-planet system is established in which the pair is captured into 2:1 MMR. By adding a third planet beyond the orbit of the reference pair and varying its mass and location, we show that the outermost planet can drive the inner planet pair to migrate from 2:1 MMR to 3:2 MMR. Increasing the mass of the outermost planet raises the probability that both the inner and outer pairs are captured into 3:2 MMR. When the outermost planet is more massive, reducing its initial semimajor axis increases the chance for the inner pair to enter 3:2 MMR, while this effect is weak for lower masses. A fourth planet at the outermost orbit shows that its migration has little influence on the configuration of the inner three planets if it migrates slowly. The sequence in which planet pair enters resonance may be a key factor leading to different system architectures. The simulation results are consistent with the observed period ratio distribution in chain resonance systems and offer an explanation for their compact orbital architecture.

     

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