Seeding life on habitable planets

人类的两大任务是寻找适合居住的行星和寻找地球以外的生命迹象。到目前为止，我们已经发现了4000多颗围绕其他恒星运行的行星，即所谓的系外行星。在这些系外行星中，有21颗与地球大小相似，并且也在宜居带围绕它们的主星运行，宜居带是一个温带地区，液态水可能能够在行星表面存活。然而，一颗行星在温带的存在只是决定一颗行星是否真正适合居住的几个标准之一。到目前为止，我们只知道宇宙中有一个地方有生命的起源和繁荣，那就是地球。为什么地球似乎是太阳系中唯一一个生命繁荣的行星，这一点至今仍未得到很好的理解，尤其是因为我们的邻居金星也在太阳周围的温带地区运行，但它的大气层和表面却不适合生命生存。像地球和金星这样的岩石行星是由月球大小的天体的剧烈碰撞形成的，碰撞产生的能量会产生熔化的高温表面，水等挥发物会从表面蒸发到太空中。那么究竟是什么让地球上的生命变得更友好呢？有一种理论反驳说，彗星的撞击——太阳系形成过程中冰冷的残余物——在地壳冷却和凝固时，向年轻的地球表面输送了大量的水和对生命有益的（富含碳的）成分。这为地球补充了生命开始所需的成分。然而，这就提出了一个问题，即彗星在为*所有*可能宜居的行星播种适宜生命的成分方面所起的作用。彗星撞击是形成宜居行星的重要过程吗？如果是这样，那么在其他恒星周围形成的彗星是否也携带着富含碳和对生命有利的物质呢？这项研究将通过使用最先进的计算化学和气候模型，结合对彗星和其他恒星周围的外彗星形成区域的最新观测，研究彗星在*所有*可能适合居住的行星上播种生命的作用，从而仔细审查可居住性所需的标准。我和我的团队将确定I)系外行星形成系统中的彗星构建材料是否为孕育生命所需的富含有机物质的普遍载体，ii)彗星对岩石系外行星大气的影响是否是一种可观察到的现象。这项研究的结果将为可居住行星的共性提供强有力的约束，并提供一套诊断方法，用于使用下一代望远镜寻找彗星撞击的证据，这些望远镜将瞄准潜在的可居住系外行星。这项研究将修订“宜居性”的定义，并将在已经提出的生物特征之外提供可居住性的大气诊断。

Two of the great quests of humankind are the hunt for habitable planets and the search for the signatures of life beyond planet Earth. To date, we have discovered more than 4000 planets orbiting other stars, the so-called exoplanets. Of these exoplanets, 21 have a similar size to Earth and also orbit their host star in the habitable zone, a temperate region where liquid water may be able to survive on the planet surface. However, a planet's presence within this temperate zone is only one of several criteria that determines whether or not a planet is truly habitable. So far, we know of only one place in the universe where life has begun and thrived, planet Earth. It is still not well understood why the Earth is seemingly the only planet in the Solar System where life has flourished, especially because our neighbour, Venus, also orbits within the temperate region around the Sun, yet has an atmosphere and surface that is not friendly for life. Rocky planets like the Earth and Venus are formed from cataclysmic collisions of moon-sized bodies, the energy from which would have created a molten, hot surface from which volatiles, such as water, would have boiled away to space.So what happened to make the Earth life friendly? One theory that has stood against scrutiny is that impacts from comets, icy leftovers from the formation of the Solar System, delivered a substantial volume of water and life-friendly (carbon-rich) ingredients to the surface of the young Earth when the crust cooled and solidified. This replenished the planet with the ingredients needed for life to begin. However, this then raises questions on the role of comets in seeding *all* potentially habitable planets with life-friendly ingredients. Are cometary impacts a vital process in the formation of a habitable planet? If so, are comets formed around other stars also carriers of carbon-rich and life-friendly material?This research will scrutinise the criteria needed for habitability by investigating the role of comets in seeding life on *all* potentially habitable planets, using state-of-the-art computational chemical and climate models, in conjunction with state-of-the-art observations of comets and exo-comet forming regions around other stars. My team and I will determine i) whether or not the comet-building material in exoplanet-forming systems are universal carriers of organic-rich material needed to seed life, and ii) whether or not cometary impacts on the atmospheres of rocky exoplanets are an observable phenomenon. The outputs from this research will provide strong constraints on the commonality of habitable planets, and provide a suite of diagnostics to search for evidence of cometary impacts using next generation telescopes that will target potentially habitable exoplanets. This research will revise the definition of "habitability" and will provide atmospheric diagnostics of habitability beyond the already proposed biosignatures.