Investigating the characterisation of exoplanet atmospheres through retrieval with advanced chemistry modelling

最近发现的Trappist-1系统表明，小型多岩石系外行星的特征是触手可及的，特别是在低质量恒星周围。NASA凌日系外行星调查卫星(TESS，于2018年4月发射)将识别围绕一系列恒星类型的明亮恒星运行的更小的系外行星。这些遥远的新世界可能蕴藏着生命，这一想法令人着迷，并为我们提供了一个强大的动机，以提高我们对大气过程的理解，大气过程可能对生命的出现起作用，和/或可能影响对揭示生命存在的生物特征的检测。在这些过程中，辐射和光化学，即研究分子和原子如何与光相互作用来影响大气结构和组成，是行星大气模拟中的一个主要过程。该项目致力于研究行星大气中的辐射和光化学过程，特别是在类地系外行星上的应用。目前的观测技术，以及即将发射的詹姆斯·韦伯太空望远镜(JWST)，有利于探测炎热和/或明亮的系外行星。随着越来越小和更冷的系外行星的发现，以及极大望远镜(ELT)对它们可能的描述，有云雾大气的陆地世界将不得不越来越多地被考虑。辐射和光化学过程，包括化学烟雾和/或云的性质，对大气结构和组成以及这些行星的光谱特征起着关键作用。考虑到这些过程是至关重要的，不仅对于描述未来地球2.0上潜在的生物特征，而且对于对系外行星大气的一般理解以及对现在和未来的观测的解释。埃克塞特大学的天体物理学小组目前处于系外行星探测和大气建模的前沿，最近开发了最先进的工具来解决这些问题。ATMO是一个灵活的一维辐射-对流化学动力学模型，包括对辐射传输的完整处理以及复杂和多功能的化学网络。Atmo已经成功地应用于棕矮星和热木星的大气中，但它仍然缺乏对类地行星的大气化学进行建模的能力。此外，英国气象局统一模型(UM)，一种通常用于预测地球天气和气候的通用环流模型(GCM)，已被用于研究类地行星和气态巨型系外行星。这个项目主要涉及使用这些当地最先进的工具，以及它们进一步适应对各种类地行星进行建模，并模拟它们未来可能进行的观测。学生需要的背景主要是物理和数学。化学和/或数值模拟的背景是受欢迎的，但不是强制性的。

The recent discovery of the TRAPPIST-1 system shows that the characterisation of small, rocky exoplanets is within reach, particularly around low-mass stars. The NASA Transiting Exoplanet Survey Satellite (TESS, launched in April 2018) will identify even smaller exoplanets orbiting bright stars with a range of stellar types. The idea that those distant new worlds could harbour Life is fascinating and provides a strong motivation to improve our understanding of the atmospheric processes which may play a role on the emergence of Life and/or could affect the detection of biosignatures revealing its presence. Among those processes, radiation and photochemistry, namely the study of how molecules and atoms interact with light to influence the atmospheric structure and composition, is a major one in planetary atmosphere modelling.This project is devoted to the study of the radiative and photochemical processes in planetary atmospheres, with applications specifically to terrestrial exoplanets. Current observation techniques, along with the soon-to-launch James Webb Space Telescope (JWST), favour the detection of hot and/or bright exoplanets. With the discovery of ever smaller and colder exoplanets, and their possible characterisation with the Extremely Large Telescope (ELT), terrestrial worlds with cloudy/hazy atmospheres will have to be increasingly considered. Radiative and photochemical processes, including the properties of chemical haze and/or clouds, are playing a key role on the atmospheric structure and composition and on the spectroscopic signatures of these planets. Taking those processes into account is of key importance, not only for the characterization of potential biosignatures on the future Earth 2.0, but also for the general understanding of exoplanet atmospheres and the interpretation of present and future observations.The Astrophysics group at the University of Exeter is currently at the forefront of exoplanetary detection and atmosphere modelling and has recently developed state-of-the art tools to address these issues. ATMO is a flexible 1D radiative-convective chemical kinetic model including a complete treatment of radiative transfer as well as sophisticated and versatile chemical networks. ATMO has already been applied successfully to the atmospheres of brown dwarfs and hot Jupiters but it still lacks the ability to model the atmospheric chemistry of terrestrial planets. Also, the UK Met Office Unified Model (UM), a General Circulation Model (GCM) usually used to predict Earth's weather and climate, has been adapted for the study of both Earth-like and gas giant exoplanets. This project will mainly involve the use of these local state-of-the-art tools and their further adaptation to modelling a large variety of terrestrial planets and simulate their possible future observations.The required background for the student is primarily in physics and mathematics. Backgrounds in chemistry and/ or numerical simulations are welcome, but not mandatory.