One of the most debated issues in Solar System studies of the last decade is whether there is a massive body hidden beyond the orbit of Neptune, known as ‘Planet Nine’ (P9). Invisible to direct observations so far, its presence has been hinted at by its effects on the dynamics of the Trans-Neptunian Objects (TNO) population, and many works have been published to support or disprove the significance of such observations. Konstantin Batygin and colleagues stand on the former side of the argument by modelling a very specific class of TNO: eccentric objects that cross the orbit of Neptune (30 au) but have semi-major axis > 100 au and an inclination < 40°. Around 30 such TNO have been discovered so far.

These objects are dynamically unstable, with a dynamical lifetime of around 100 Myr, thus they must be the product of a continuous gravitational forcing on the outer TNO population. The authors perform two sets of N-body simulations that include perturbers both internal and external to the Solar System — respectively the giant planets and galactic effects such as tides and stellar flybys. One set includes P9 effects (set as a body of 5 Earth masses, a semi-major axis of 500 au, eccentricity of 0.25, and 20° inclination) and the other does not. Batygin and co-authors find that the set with P9 ‘on’ explains the presence of the observed TNO sub-population naturally and reproduces their properties better.

过去十年太阳系研究中争论最多的问题之一是海王星轨道之外是否隐藏着一个巨大的天体，即“第九行星”（P9）。迄今为止，直接观测无法看到它的存在，但它对海王星外天体（TNO）群体动力学的影响暗示了它的存在，并且已经发表了许多著作来支持或反驳此类观测的重要性。 Konstantin Batygin 和同事站在了争论的前一边，他们对一类非常特殊的 TNO 进行了建模：穿过海王星轨道（30 个天文单位）但半长轴 > 100 个天文单位且倾角 < 40° 的偏心天体。迄今为止已发现约30个这样的TNO。

这些天体动态不稳定，动态寿命约为 100 Myr，因此它们一定是 TNO 外部种群持续引力作用的产物。作者进行了两组 N 体模拟，其中包括太阳系内部和外部的扰动因素，分别是巨行星和银河效应，例如潮汐和恒星飞越。一组包括 P9 效应（设置为 5 个地球质量的物体、500 au 的半长轴、0.25 的偏心率和 20° 倾角），而另一组则不包括。 Batygin 和合著者发现，P9“开启”的集合自然地解释了观察到的 TNO 亚群的存在，并更好地再现了它们的特性。