The Moons of Uranus May Hold the Key to Finding Missing Planets
The Moons of Uranus May Hold the Key to Finding Missing Planets
天王星的卫星可能是寻找失踪行星的关键
We have an idea of what the solar system’s past was like: It was violent and chaotic. However, we are still studying how violent it was. Current models suggest that at some point after their formation, the giant planets went through a phase of such extreme instability that one or even two bodies the size of Uranus or Neptune were ejected into interstellar space. If that scenario occurred, we may find clues in the most unexpected places in the solar system, such as the moons of Jupiter and, especially, those of Uranus.
我们对太阳系的过去有一定了解：那是一个暴力且混乱的时期。然而，我们仍在研究它究竟有多暴力。目前的模型表明，在巨行星形成后的某个阶段，它们经历了一段极度不稳定的时期，以至于一颗甚至两颗天王星或海王星大小的天体被抛射到了星际空间。如果这种情况确实发生过，我们或许能在太阳系中最意想不到的地方找到线索，例如木星的卫星，尤其是天王星的卫星。
A recent article published in Icarus analyzed 122 possible scenarios of such instability to assess how the satellite systems of the “left behind” planets would have reacted. The researchers concluded that it would be extremely difficult to explain the current characteristics of Uranus’ moons without some episode of violent instability. And that type of instability only appears in models where more giant planets existed than we see today.
最近发表在《伊卡洛斯》（Icarus）杂志上的一篇文章分析了 122 种可能的不稳定情景，以评估“留存”行星的卫星系统会如何反应。研究人员得出结论：如果没有经历过某种剧烈的不稳定事件，就极难解释天王星卫星目前的特征。而这种不稳定性只出现在那些巨行星数量多于我们今天所见数量的模型中。
Most likely, the authors point out, the moons of Uranus were destabilized at least twice in the past: First by the impact that tilted the planet, and then by close encounters between giant planets during the instability. That chaos, fueled by the presence of one or more planets that were later ejected, would have destroyed and rebuilt the system of moons to what we see today.
作者指出，最有可能的情况是，天王星的卫星在过去至少经历过两次不稳定：第一次是导致天王星倾斜的撞击，第二次是巨行星在不稳定期间的近距离接触。这种由一颗或多颗后来被抛射的行星所引发的混乱，摧毁并重塑了卫星系统，形成了我们今天所看到的模样。
The Solar System and Chaos
太阳系与混乱
Jupiter, Saturn, Uranus and Neptune did not always have their current positions in the solar system. According to the planetary-instability model, they were born a little closer to the Sun and closer together. After millions of years, they migrated towards their current orbits.
木星、土星、天王星和海王星在太阳系中并非一直处于当前的位置。根据行星不稳定模型，它们诞生时距离太阳更近，彼此之间也靠得更近。经过数百万年的演变，它们迁移到了现在的轨道上。
But there are details of this model that do not fit with the observations. For one thing, the current orbits of Jupiter and Saturn are eccentric, while there are specific structures such as the Kuiper belt that seemingly should have prevented Neptune from moving into its current position. In the simulations, the planets did not reach where they are today.
但该模型中存在一些与观测结果不符的细节。首先，木星和土星目前的轨道是偏心的；其次，像柯伊伯带这样的特定结构似乎本应阻止海王星移动到当前位置。在模拟中，行星并没有到达它们今天所在的位置。
It is therefore possible that the solar system at one point had more planets, and these were the ones that “pushed the others.” Under this hypothesis, the puzzle of the solar system fits better. The problem is, those bodies, if they existed, are gone—they were ejected and left no physical traces or fragments. This leaves the idea of missing planets in the realm of hypotheses, waiting for sufficient evidence to be accumulated to confirm it.
因此，太阳系在某个时期可能拥有更多的行星，正是这些行星“推动了其他行星”。在这个假设下，太阳系的谜题更容易解释。问题在于，如果这些天体曾经存在，它们现在已经消失了——它们被抛射出去，没有留下任何物理痕迹或碎片。这使得“失踪行星”的想法仍处于假设阶段，等待积累足够的证据来证实。
The Unusual Moon
不寻常的卫星
The new Icarus study tested the missing planets hypothesis using the moons of Uranus as direct evidence. It used a total of 122 solar system evolution simulations. In 85 percent of the scenarios, the Uranus moon system collapsed. Only in a handful of scenarios did its moons survive, and, in all of them, the hypothesis of lost and ejected planets fit very well.
这项新的《伊卡洛斯》研究利用天王星的卫星作为直接证据，检验了失踪行星的假设。研究总共使用了 122 次太阳系演化模拟。在 85% 的情景中，天王星的卫星系统崩溃了。只有在少数情景中，其卫星得以幸存，而在所有这些情景中，失踪和被抛射行星的假设都非常吻合。
The report points to Miranda, the smallest moon in Uranus’ major system. Astronomers consider it to be the most unusual in the solar system. It is patchy, as if sewn together from scraps, too icy for its size, and quite small considering the rest of Uranus’ moons. It is also geologically active.
报告特别提到了米兰达（Miranda），它是天王星主要卫星系统中最小的一颗。天文学家认为它是太阳系中最不寻常的卫星。它表面斑驳，看起来像是用碎片缝合在一起的；相对于其体积而言，它太冰冷了，而且考虑到天王星的其他卫星，它显得非常小。此外，它在地质上也是活跃的。
Astronomers think that Miranda is the debris of a larger body. The study reinforces that idea and proposes that it is the clearest example of traces of planetary instability.
天文学家认为米兰达是一个更大天体的残骸。这项研究加强了这一观点，并提出它是行星不稳定痕迹最明显的例子。
This work does not yet solve the mystery of the missing planets, but it does show that the moons could serve as witnesses to the chaos of the solar system. Such data, along with other independent data on unusual structures such as the Trojans, Jupiter’s asteroids, or the mere presence of the Oort cloud, will someday tell what happened to those missing bodies—if they really existed.
这项工作虽然尚未解开失踪行星的谜团，但它确实表明，卫星可以作为太阳系混乱历史的见证者。这些数据，连同关于特洛伊小行星群、木星小行星或其他如奥尔特云存在等不寻常结构的其他独立数据，终有一天会揭示那些失踪天体到底发生了什么——如果它们确实存在的话。
On the other hand, a dedicated mission to Uranus, such as the one NASA and ESA are discussing for the 2040s, could confirm whether Miranda is indeed a reconstructed body after the chaos. If it is, then the moons could be the key to understanding how many worlds the solar system really had.
另一方面，一项针对天王星的专门任务（例如 NASA 和 ESA 正在讨论的 2040 年代任务）可能会证实米兰达是否确实是在混乱之后重组的天体。如果是这样，那么卫星可能就是理解太阳系曾经拥有多少个世界的关键。