行星V

行星V 是一颗由美国国家航空航天局的科学家约翰·钱伯斯（英语：John Chambers (scientist)）和杰克·利斯奧爾（英语：Jack J. Lissauer）假设存在于火星和小行星带之间的行星。他们在2002年3月11日至15日之间举办的第33届月球与行星科学大会上提出了这个假说。

他们认为，40亿年前，有一颗类地行星沿着一个离心率较高且不稳定的轨道围绕太阳运行。他们把这颗行星命名为行星V，并且认为这颗行星的消失和冥古宙的后期重轰炸期有关。[1] [2]

假说

行星V假设认为，行星形成时代产生了5颗类地行星。行星V一开始位于火星和小行星带之间的低离心率轨道上运行，半长轴在1.8至1.9AU之间。虽然寿命很长，但在6亿年的尺度上不稳定。最终，其他内行星的扰动使行星V进入了高离心率轨道，越过了内小行星带。与行星V近距离相遇后，小行星被抛到穿越火星轨道和轨道共振的位置上，很多穿越了地球轨道，暂时提高了月球受撞击率。这过程一直持续到行星V消失，可能是在进入ν6长期共振后撞击了太阳。[3]

测试与结果

作为对假说的初步测试，钱伯斯和利斯奧爾进行了36次计算机模拟，使用各种参数确定行星V的初始轨道和质量对太阳系的影响。结果发现，当行星V的初始半长轴从1.8AU增加到1.9AU时，行星V消失的平均时间从1亿年增加到4亿年。在质量为0.25倍火星的情形下，与当前太阳系一致的结果最常见；质量较大的行星V往往会导致行星间碰撞。总的来说，有三分之一的模拟可认为是成功的：行星V移动而没有撞到别的行星。对月球撞击率的测试增加了测试粒子，行星V进入内小行星带后，地球交叉轨道上的粒子数下降后又增加，这种模式与LHB一致。这些结果在第33届月球与行星科学大会上做了介绍.[3]

钱伯斯发表在《伊卡洛斯》（2007）的文章中，报告了96次模拟结果。在四分之一的模拟中，行星V被抛出太阳系或撞击太阳，没有与其他内行星碰撞。行星V的质量小于火星的0.25倍时这种情形最常见。其他模拟则不成功，行星V或者经历10亿年幸存下来，或者与内行星发生了碰撞。[4]

Ramon Brasser、Alessandro Morbidelli (2011)对行星V假说进行了研究，他们计算出，要产生LHB的撞击，行星V必须清除95%的轰炸前主小行星带或98%的内小行星带（半长轴小于2.5AU）。研究发现，要使质量为0.5倍火星的行星V清除95%的主小行星带，要在穿越整个小行星带的轨道上停留3亿年，这不见于任何模拟；行星V通常会进入穿越地球的轨道，因此之前的动态寿命很短。在百分之几的模拟中，行星V在小行星带停留的时间足够长，产生了LHB；但从内小行星带产生LHB需要其初始质量是其他小行星带的4–13倍、轨道密度是10–24倍。[5]

Brasser & Morbidelli还研究了行星V扰乱类地行星之间的假想小行星带而导致LHB的假说。作者指出，由于目前没有观测到其残余，对假说造成了很大限制，要求小行星带在行星V之前就耗尽了99.99%。虽然在金星-地球带的模拟中有66%符合目前的太阳系情况，但在地球-火星带的模拟中，稳定性较高以至不存在这种情况。Morbidelli & Brasser总结说，地球-火星带不太可能包含大量星群。行星V虽然可通过破坏大质量的金星-地球带单独产生LHB，但行星形成模型中没有产生这些带的显著差异。[5]

替代版本

最近有人提出，行星V撞击了火星，形成了北半球盆地，作为晚期重轰炸期的一种解释。这种撞击碎片的大小分布与小行星带不同，大天体的比例较小，因此巨型撞击盆地的数量相对于陨石坑的数量较少。[6][7]

参见

参考文献

  1. ^ Long-Destroyed Fifth Planet May Have Caused Lunar Cataclysm. Space.com. [2006-11-09]. （原始内容存档于2002-04-08）. 
  2. ^ A NEW DYNAMICAL MODEL FOR THE LUNAR LATE HEAVY BOMBARDMENT (PDF). Chambers and Lissauer, NASA Ames. [2006-11-09]. （原始内容存档 (PDF)于2012-09-08）. 
  3. ^ 3.0 3.1 Chambers, J. E.; Lissauer, J. J. A New Dynamical Model for the Lunar Late Heavy Bombardment (PDF). Lunar and Planetary Institute Science Conference Abstracts. 2002, 33: 1093 [2012-03-16]. Bibcode:2002LPI....33.1093C. （原始内容存档 (PDF)于2012-02-17）. 
  4. ^ Chambers, J. E. On the stability of a planet between Mars and the asteroid belt: Implications for the Planet V hypothesis. Icarus. 2007, 189 (2): 386–400. Bibcode:2007Icar..189..386C. doi:10.1016/j.icarus.2007.01.016. 
  5. ^ 5.0 5.1 Brasser, R; Morbidelli, A. The terrestrial Planet V hypothesis as the mechanism for the origin of the late heavy bombardment. Astronomy & Astrophysics. 2011, 535: A41. Bibcode:2011A&A...535A..41B. doi:10.1051/0004-6361/201117336. 
  6. ^ Minton, D. A.; Jackson, A. P.; Asphaug, E.; Fassett, C. I.; Richardson, J. E. Debris from Borealis Basin Formation as the Primary Impactor Population of Late Heavy Bombardment (PDF). Workshop on Early Solar System Impact Bombardment III. 2015, 1826: 3033 [2023-11-16]. Bibcode:2015LPICo1826.3033M. （原始内容存档 (PDF)于2023-10-13）. 
  7. ^ Minton, David A.; Richard, James E.; Fassett, Caleb I. Re-examining the main asteroid belt as the primary source of ancient lunar craters. Icarus. 2015, 247: 172–190. Bibcode:2015Icar..247..172M. S2CID 55230320. arXiv:1408.5304 可免费查阅. doi:10.1016/j.icarus.2014.10.018. 


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