The role of giant impacts in the formation of the outer solar system and exosystems

项目背景：巨大的撞击（行星大小的物体之间的碰撞）是行星形成过程中最剧烈的事件。在几个小时内，形成比地球质量更大的天体的碰撞释放出比太阳更多的能量。碰撞体的大部分冰和/或岩石地幔被融化和蒸发，施加的巨大扭矩可以使碰撞后的物体快速旋转。最大物体的质量可能增加或减少，这取决于从系统中喷出的物质的数量。撞击可以从根本上改变行星的演化轨迹，因为大气、地壳、地幔和地核的比例都发生了变化，卫星系统也可以形成。例如，巨大的撞击被认为是造成天王星高倾角、水星高密度和月球形成的原因。撞击后天体的热和旋转状态是如此极端，以至于很大一部分撞击会产生索内斯蒂亚，这是最近理论提出的一类行星物体（Lock & Stewart, 2017）。Synestias是超过自转极限的天体，自转极限定义为行星赤道速度等于圆形开普勒轨道的角动量。撞击后的索内斯蒂亚通常比较冷的行星大许多倍，并形成甜甜圈形状的结构（图1）。索内斯蒂亚不同的动力学和热力学状态对月球的形成、地核的形成和行星内挥发物的分布具有重要意义。在我们的太阳系中，只有有限的巨大撞击记录，但系外行星的观测正在彻底改变这一领域。最近的太空望远镜，如开普勒和TESS，以及多个地面调查，正在显著增加已知系外行星的数量。系外系统有一系列的结构和密度变化很大的宿主行星，因此见证了巨大撞击和行星形成的许多可能结果。此外，随着我们发现越来越多的系外行星并开始观察年轻恒星周围的行星，我们可能很快就会在巨大撞击的直接后果中发现行星体。对巨大撞击的研究，以及对这些观测结果的解释，主要集中在我们内太阳系形成过程中预期的撞击上。关于更大质量行星之间碰撞的研究很少，尽管这种碰撞可能有助于形成外太阳系。此外，我们的太阳系可能不是一个典型的行星系统。它缺乏在其他恒星周围发现的最常见的行星：超级地球和迷你海王星（半径介于地球和海王星之间的行星）。许多这样的行星都有大量的冰（例如水）和/或气体。很少有研究探索更大质量的陆地和/或挥发物丰富的天体之间的影响，目前，我们不了解宇宙中可能最频繁的行星碰撞的结果。项目目标和方法该项目将探索碰撞如何塑造我们太阳系和系外系统中的行星。利用最先进的反碰撞模拟，成功的候选人将研究富水体之间碰撞的动态和热力学结果的范围，并开发将结果与撞击参数联系起来的“比例定律”。虽然撞击的频率和能量在不同的行星形成模型中是不同的，但这是一种常见的现象。通过分析每个场景中撞击的频率和参数，学生将确定在不同情况下，巨大的撞击会如何塑造行星系统。此外，通过确定撞击后天体和索内斯蒂亚的观测特征，该项目将旨在提供必要的工具，以确定……

Project Background:Giant impacts (collisions between planet-sized bodies) are the most violent events in planet formation. For a few hours, collisions that form greater than Earth-mass bodies release more energy than the Sun. Large fractions of the icy and/or rocky mantles of the colliding bodies are melted and vaporised, and the huge torques exerted can leave the post-impact body rapidly rotating. The mass of the largest body may either increase or decrease, depending on the amount of material ejected from the system. Impacts can fundamentally alter the trajectory of a planet's evolution, as the ratios of atmosphere, crust, mantle and core all change and systems of moons can be formed. For example, giant impacts are thought to be responsible for the high obliquity of Uranus, the high density of Mercury, and the formation of Earth's Moon.The thermal and rotational states of post-impact bodies are so extreme that a significant fraction of impacts produce synestias, a recently theorisedclass of planetary object (Lock & Stewart, 2017). Synestias are bodies that exceed the corotation limit, defined as the angular momentum at which a planet's equatorial velocity equals that of a circular Keplerian orbit. Post-impact synestias are typically many times larger than cooler planets and form donut-shaped structures (Figure1). The different dynamical and thermodynamical states of synestias have significant implications for moon formation, core formation, and the distribution of volatiles within planets.Only a limited record of giant impacts remains in our solar system, but exoplanet observations are revolutionising this field. Recent space telescopes, such as Kepler and TESS, along with multiple ground-based surveys, are dramatically increasing the number of known exoplanets. Exosystems have a range of architectures and host planets with widely varying densities, and so bear witness to the many possible outcomes of giant impacts and planet formation in general. Furthermore, as we find more and more exoplanets and begin to observe planets around younger stars, we may soon detect planetary bodies in the immediate aftermath of giant impacts.Studies of giant impacts, with which these observations are interpreted, have largely focused on the impacts expected during formation of our inner solar system. There has been little work on collisions between more massive planets, even though such impacts could have helped shape the outer solar system. In addition, ours may not be a typical planetary system. It lacks the most common planets found around other stars: super-Earths and mini-Neptunes (planets with radii between Earth and Neptune). Many of these planets have large ice (e.g., water) and/or gas fractions. Few studies have explored impacts between more massive terrestrial and/or volatile-richbodies and, at present, we do not understand the outcome of what are likely the most frequent planetary collisions in the universe.Project Aims and MethodsThis project will explore how collisions shape the planets in our solar system and in exosystems. Using state-of-the-artgiantimpact simulations, the successful candidatewill investigate the range of dynamic and thermodynamic outcomes of collisions between water-rich bodies, and develop 'scaling laws' that relate the outcomes to the impact parameters. Although generally a common occurrence, the rates and energies of impacts vary between different planet formation models. By analysing the frequency and parameters of impacts in each scenario, the student will ascertain how giant impacts would sculpt planetary systems in different cases. Furthermore, by determining the observational signatures of post-impact bodies and synestias, the project will aim toprovide the tools necessary to identify...