图55巨蟹座：一个测试关于行星形成的众多猜想的实验室
55 Cancri: A Laboratory for Testing Numerous Conjectures about Planet Formation
摘要：
目前有五颗行星被认为围绕着55 Cnc的主星运行，但在最外层的两颗行星之间存在着5 AU的差距。这个间隙引起了相当大的兴趣，因为它可能包含一个或多个低质量的行星，其存在并不矛盾的长期轨道稳定性分析，事实上，它是预期根据“包装行星系统”假说和经验提丢斯-波德关系最近提出的55 CNC。此外，第二大行星只是第二远，它的轨道非常接近恒星。它的轨道是所有行星中最圆的，似乎与第三颗行星的轨道几乎但不完全相同，这使人怀疑是否发生过内行星的迁移或共振捕获，并支持通过核心吸积过程“原位”形成巨型行星形成的想法。随着更多的观测结果的出现，所有上述想法都将在未来几年在这个天然实验室中得到检验。这个机会与一个物理模型相结合，该模型将观察到的行星的轨道与原始原行星盘的结构联系起来，该结构在原恒星坍缩的早期阶段形成。仅使用55 Cnc的5颗观测到的行星，该模型预测其原行星盘的表面密度轮廓随距离$R$的变化精确地为$Σ(R)\propto R^{-3/2}$，这也被发现为最小质量的太阳星云。尽管有这种相似性，但55 CNC的盘比太阳星云更小，更重，旋转支撑更少，因此与我们自己的太阳系相比，这个系统代表了一种不同的多行星形成模式。
英文摘要：
Five planets are presently believed to orbit the primary star of 55 Cnc, but there exists a large 5 AU gap in their distribution between the two outermost planets. This gap has attracted considerable interest because it may contain one or more lower--mass planets whose existence is not contradicted by long-term orbit stability analyses, in fact it is expected according to the "packed planetary systems" hypothesis and an empirical Titius--Bode relation recently proposed for 55 Cnc. Furthermore, the second largest planet is just the second farthest and it orbits very close to the star. Its orbit, the most circular of all, appears to be nearly but not quite commensurable with the orbit of the third planet, casting doubt that any migration or resonant capture of the inner planets has ever occurred and lending support to the idea of "in--situ" giant planet formation by the process of core accretion. All of the above ideas will be tested in the coming years in this natural laboratory as more observations will become available. This opportunity presents itself in conjunction with a physical model that relates the orbits of the observed planets to the structure of the original protoplanetary disk that harbored their formation at the early stages of protostellar collapse. Using only the 5 observed planets of 55 Cnc, this model predicts that the surface density profile of its protoplanetary disk varied with distance $R$ precisely as $Σ(R)\propto R^{-3/2}$, as was also found for the minimum--mass solar nebula. Despite this similarity, the disk of 55 Cnc was smaller, heavier, and less rotationally supported than the solar nebula, so this system represents a different mode of multi--planet formation compared to our own solar system.
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