The Descent of Planets

在太阳系外已经发现了350多颗行星。这些“系外行星”中的大多数在质量上与木星相似，但它们的轨道离中心恒星更近。行星的形成是很常见的，至少有5%的类太阳恒星在近轨道上有一颗木星质量的行星。更多的恒星可能有行星，这些行星太小或离主星太远，用目前的技术无法探测到。我们观察到的星系有着惊人的多样性，与我们的太阳系非常不同。观测无法追溯行星形成的全部历史，但确实提供了围绕年轻恒星运行的尘埃气体盘的早期阶段或行星系统或碎片盘形成后的后期阶段的快照。因此，我们无法通过观测将行星形成的早期和晚期联系起来，也不知道如此多样化的太阳系外系统是如何产生的，也不知道是什么决定了太阳系的发展类型。解决方案是建立一个可测试的模型，该模型可以将范围广泛的原行星盘（包括但不限于观察到的原行星盘）进化到最终的行星系统或碎片盘。虽然模型应该再现所有观察到的最终状态，但产生的许多最终系统将没有被观察到。由于数值限制和不完整/未知的物理学，天体物理学界未能建立这样一个完整的行星形成模型。为了使行星形成问题更容易处理，行星形成过程往往被分成不同的阶段，然后单独加以处理。这种方法已经取得了一些成功，例如，大型（~100公里）的星子（行星的组成部分）成长为大约月球质量的原行星（行星的前身），并且简化的模拟继续从假设的原行星初始分布中创建行星系统。然而，我们自己的太阳系的重要特性没有得到解释，比如岩石行星的近圆形轨道，也没有太阳系外行星和碎片盘的多样性。在接下来的五年里，我将开发复杂而现实的行星形成过程的计算机模型。特别是，我将展示星子如何从公里大小的岩石成长为月球大小的原行星，并最终形成整个太阳系或碎片盘。我将制作一份太阳系的目录，以便与观测结果进行比较。这项研究还将解释我们自己的太阳系尚未完全了解的细节（岩石行星的形成速度和水的输送）。最终，我将能够确定我们的太阳系是独一无二的，还是在整个银河系中有许多类似的星系。了解太阳系和类地行星是如何形成的挑战是非常令人兴奋的，因为我们知道这些是生命可以发展的地方。研究行星的形成对社会有广泛的好处——几千年来，人们一直试图了解地球的起源和它在太阳系中的位置，以及地球是独特的还是仅仅是散布在银河系和宇宙中的大量类似行星中的一颗。从更实际的角度来看，复杂系统的复杂数值模拟广泛适用于工业，这些可转移的技能将由学生在大学环境中学习。从更长远的角度来看，了解我们太阳系的碰撞演化的好处包括防御灾难性的小行星撞击。

Over 350 planets have been discovered outside our solar system. Most of these 'extrasolar planets' are similar in mass to Jupiter but have orbits that are much closer to their central star. Planet formation is common, with at least 5% of all Sun-like stars having a Jupiter mass planet in close orbit. Many more stars may have planets that are too small or too far from the host star to be detected with current techniques. The systems that are observed are surprisingly diverse and very different from our solar system. Observations cannot trace the full history of planet formation, but do provide snapshots of either early stages of a dusty gas disk orbiting a young star or the late stages after planetary systems or debris disks have formed. As a result, we cannot observationally connect the early and late stages of planet formation, and it is not known how such a diversity of extrasolar systems arises, or what determines the type of solar system that develops. The solution is to build a testable model that can evolve a broad range of protoplanetary disks (including, but not limited to, the observed protoplanetary disks) through to final planetary systems or debris disks. While the model should reproduce all of the observed end states, many of the final systems produced will not have been observed. Such a complete model of planet formation has eluded the astrophysics community because of numerical limitations and incomplete/unknown physics. In order to make the problem of planet formation more tractable the planet formation process is often divided into separate stages, which are then tackled in isolation. This method has had some success, for example, large (~100 km) planetesimals, the planet building blocks, grow into protoplanets (planetary precursers) of about a lunar mass, and simplified simulations go on to create planetary systems from an assumed initial distribution of protoplanets. However, important properties of our own Solar System are not explained, such as the near circular orbits of the rocky planets, nor the diversity of the extrasolar planets and debris disks. During the next five years I will develop sophisticated and realistic computer models of the planet formation process. In particular, I will show how planetesimals grow from km-sized rocks into lunar-sized protoplanets and ultimately entire solar systems or disks of debris. I will produce a catalogue of solar systems that can be compared with observations. This research will also explain details of our own solar system that have yet to be fully understood (rate of rocky-planet formation and water delivery). Ultimately I will be able to determine whether our solar system is unique, or has many counterparts throughout the Galaxy. The challenge to understand how solar systems and terrestrial planets form is very exciting, for these are the places where we know life can develop. Studying planet formation has a wide benefit to society - for thousands of years people have sought to understand the origin of the Earth and its place in the solar system, and whether the Earth is unique or merely one of a vast number of similar planets scattered throughout the Galaxy and Universe. On a more practical note, sophisticated numerical simulations of complex systems are widely applicable to industry, and these transferable skills will be learned by students in a University environment. Much longer term benefits of understanding the collisional evolution of our solar system include defence against catastrophic asteroid impacts.