Inverse Modelling Methods for the Characterisation of Exoplanetary Atmospheres

自从2009年开普勒太空望远镜发射以来，现在的结果表明，五分之一的类太阳恒星可能有可居住的行星（Petigura et al., 2013），这可能对天体生物学领域和在宇宙其他地方寻找生命有着惊人的影响。在过去的20年里，我们从探测行星，到测量体积特性（如密度），再到最近，对这些行星的大气层特征进行了描述。这样的大气研究对于评估宜居性的长期目标至关重要，因为即使在我们自己的太阳系内，火星和金星的案例研究也表明，由于它们的大气构成，原本“宜居”的行星如何被证明是不适宜居住的。我们现在面临着向低质量世界的持续推进，这使得探测真正类似地球的大气层的前景诱人地接近于实现。过去许多试图真正描述系外行星大气特征的尝试中，有一个问题一直困扰着人们，那就是人们对简并性知之甚少。例如，当我们观察凌日行星-恒星系统的光时，某些光谱特征被解释为由于给定分子的吸收，也可以用不同的大气温度结构来解释。在过去，单一的模型一直运行，直到找到一个适合的数据，而没有对其他可以解释数据的大气参数组合进行全面的调查。为了避免这种情况，所谓的“大气检索”模型已经被开发出来（最初是由我的导师Madhusudhan博士开发的），它运行了数百万个模型，以充分探索观察到的行为的所有可能的解释。这些方法采用观测数据并利用它来推断大气的特性（通常称为“逆模型”）。我目前的研究重点是利用上述大气检索方法对系外行星透射光谱进行理论建模。我的工作包括研究可以包含在这些模型中的新物理现象，以便从现有的系外行星透射光谱中提取更多的信息内容。在即将到来的高精度系外行星光谱时代（例如，詹姆斯·韦伯太空望远镜），这将被证明是越来越重要的，因为数据的质量将足以以前所未有的细节描绘这样的世界。然而，这需要对以前被忽视的物理效应进行广泛的理论探索，以确保潜在发现的最大开采可能性。

Since the launch of the Kepler Space Telescope in 2009, results now suggest that 1 in 5 Sun-like stars may have habitable planets (Petigura et al., 2013), which potentially holds startling implications for the field of Astrobiology and the search for life elsewhere in the Universe.In quick succession over the past 20 years, we have gone from planetary detection, to measuring bulk properties (such as density), to, most recently, characterisation of the atmospheres of a selection of these planets. Such atmospheric studies are vital to the long term goal of assessing habitability, as even within our own solar system the case studies of Mars and Venus demonstrate how otherwise 'habitable' planets can prove inhospitable due to their atmospheric makeup. We now face a continual drive to lower mass worlds, bringing the prospect of probing the atmosphere of a true Earth-analogue tantalisingly close to fruition.One of the issues that has plagued many past attempts to truly characterise exoplanet atmospheres is poorly understood degeneracies. For instance, when we observe the light from a transiting planet-star system, certain spectral features that have been interpreted as absorption due to a given molecule can also be explained by a different atmospheric temperature structure. In the past, single models were run until one fitting the data was found, without a throughout investigation of other combinations of atmospheric parameters that could also explain the data. To circumvent this, so called 'atmospheric retrieval' models have been developed (originally by my supervisor, Dr. Madhusudhan), which run millions of models to fully explore all the possible explanations for the observed behaviour. These take in observational data and use it to infer the properties of the atmosphere (often called 'inverse modelling').My current research focuses on theoretical modelling of exoplanet transmission spectra using the aforementioned methodology of atmospheric retrieval. My work involves investigating new physical phenomena that can be included in such models, such that a greater information content can be extracted from existing exoplanet transmission spectra. This will prove increasingly vital in the upcoming era of high-precision exoplanet spectroscopy (for instance, from the James Webb Space Telescope), as the quality of data will be sufficient to characterise such worlds in unprecedented detail. This, however, requires extensive theoretical exploration of previously neglected physical effects to ensure the maximum extraction possibility of potential discoveries.