A Pathway to the Confirmation and Characterisation of Habitable Alien Worlds

我们在宇宙中是孤独的吗？自从20世纪90年代太阳系外的第一颗行星被确认以来，我们在回答这个问题方面取得了巨大进展。然而，我们仍然无法确认一个真正的地球类似物。此外，要想真正了解宇宙中生命的起源，还必须对行星的形成、演化和可居住性有一个完整的认识，而这一切都需要对太阳系恒星有详细的了解。这是因为我们通过分析它们更明亮的宿主恒星来间接研究系外行星。例如，大多数行星的确认依赖于行星引起的宿主星星的多普勒摆动。此外，我们可以通过绘制行星经过其宿主星星时的轨道图来了解行星的动力学历史。因此，恒星表面的不均匀性可能会影响行星的解释，并可能完全淹没来自岩石世界的信号。我的研究旨在克服这些障碍。为此，我的团队从两个方面研究恒星表面：利用最先进的3D模拟和使用凌日行星来经验性地探测恒星表面。我的目标是理解和解开确认其他地球的道路上的一个基本障碍：对流引起的恒星表面不均匀性。行星的确认需要质量测量，这可以从恒星大气中吸收线的多普勒频移来确定。然而，所有的类太阳恒星都被沸腾的等离子体包裹，导致热的等离子体气泡上升到表面（引起蓝移），在那里它们冷却并落入周围区域（引起红移）。最终的结果是虚假的速度移动到1米/秒-完全淹没了地球孪生的微小信号，这只是9厘米/秒。如果磁场区域集中并抑制对流，这些变化可能会更大。随着下一代光谱仪的不断上线，我们正在进入一个技术上可行的时代，以确认地球孪生。随着2026年PLATO使命的发射，准备提供这样的候选者，以及2024年底开始的Terra Hunting Survey，装备确认这样的世界，这项工作是非常紧迫的。太阳已经向我们展示了对流不容易平衡;我们必须解开它的签名，以找到类似地球的世界。为此，我的团队使用3D磁流体动力学模拟来创建逼真的恒星模型。有了这些，我们精确地研究了对流如何改变恒星线，并致力于优化恒星降噪技术。我目前对太阳类似物的研究表明，我们可以利用恒星线的曲率来消除这种噪音，但这对较热或较冷的恒星有效吗？如果一颗星星的磁场分布不均匀，那么噪声诊断是如何进行的？哪些谱线对对流和磁场最敏感？这些是我的研究旨在回答的一些问题。当然，这些诊断只有和它们的底层模拟一样可靠。我开创了一项新技术，使用凌日行星作为探测器，首次验证了太阳以外的主序星的这些。通过从过境观测中减去过境观测，我们分离出行星后面的星光。有了这个，我们可以研究对流行为，恒星差速旋转，并确定行星轨道的3D轨迹-这是理解其形成和演化的关键特征。通过将这种技术应用于一系列系统，能够验证模拟，量化对流对行星动力学测量的影响，并有助于对行星形成和演化的更全球性的理解。通过这种双管齐下的方法，我的目标是推动天文学的前沿，走向未来可居住的外星世界的确认和表征，并帮助回答我们在宇宙中是否真的孤独。

Are we alone in the Universe? Since the confirmation of the first planets outside our solar system in the 1990s, we have made tremendous progress towards answering this question. Yet, the confirmation of a true Earth-analogue still evades us. On top of this, if we are truly to understand the origins of life in the cosmos, we must also create a complete picture of planetary formation, evolution, and habitability.However, each of these aspects necessitates a detailed knowledge of solar-type stars. This is because we study exoplanets indirectly by analysing their much more luminous host stars. For example, most planet confirmation relies on the Doppler wobble of the host star, induced by the planet. Moreover, we can learn about a planet's dynamical history from mapping its projected orbit as it transits its host star. Hence, stellar surface inhomogeneities can impact planetary interpretations, and can completely swamp the signals from rocky worlds. My research aims to overcome these hurdles. For this, my team studies stellar surfaces from a two-pronged approach: with state-of-the-art 3D simulations and using transiting planets to empirically probe stellar surfaces.I aim to understand and disentangle a fundamental barrier on the pathway to confirming other Earths: the stellar surface inhomogeneities from convection. Planet confirmation requires a mass measurement, which can be determined from the Doppler shift of the absorption lines in the stellar atmosphere. However, all Sun-like stars are enveloped in boiling plasma, causing hot bubbles of plasma to rise to the surface (inducing blueshifts), where they cool and fall down into the surrounding regions (inducing redshifts). The net result is spurious velocity shifts up to a m/s - completely swamping the tiny signal of an Earth- twin, which is a mere 9 cm/s. These shifts can be even larger if regions of magnetic field concentrate and inhibit the convection. As the next generation spectrographs continue to come online, we are entering an era where it is technologically feasible to confirm Earth-twins. With the launch of the PLATO mission in 2026, primed to provide such candidates, and the Terra Hunting Survey commencing late 2024, equipped to confirm such worlds, this work is extremely time critical.The Sun has shown us convection does not easily average out; we must disentangle its signature to find Earth-like worlds. To do this, my team uses 3D magnetohydrodynamic simulations to create realistic model stars. With these, we study precisely how convection alters stellar lines, and work to optimise stellar noise reduction techniques. My present work on Solar- analogues indicates we can use the curvature of the stellar lines to remove this noise, but will this work for hotter or cooler stars? How do noise diagnostics behave if a star has a patchy distribution of magnetic field? Which lines are most sensitive to the convection and magnetic fields? These are some of the questions my research aims to answer.Of course, these diagnostics are only as reliable as their underlying simulations. I have pioneered a new technique, using transiting planets as probes, to validate these for the first time for main-sequence stars other than the Sun. By subtracting in- from out-of-transit observations, we isolate the starlight behind the planet. With this, we can study the convection behaviour, stellar differential rotation, and determine the 3D trajectory of a planet's orbit - a key feature in understanding its formation and evolution. By applying this technique to a range of systems are able to validate the simulations, quantify the impact of convection on planetary dynamic measurements, and contribute to a more global understanding of planet formation and evolution.With this 2-pronged approach, I aim to push the frontiers of astronomy towards the future confirmation and characterisation of habitable alien worlds, and help answer whether or not we are truly alone in the Universe.