Atmospheres and Climates of Exoplanets

近年来，对系外行星的研究已经从探测它们转向表征它们的物理性质，特别是它们的大气层。这里提出的系外行星工作包括使用一些最初为地球开发的大气科学的基本工具和原理，以加深我们对系外行星上预期的大气行为多样性的理解，并阐明这些行为如何影响天文观测结果。**热系外行星**热木星/土星/海王星形成了最具特征的系外行星类别，下一代天文台和调查，如TESS， JWST和ECho，将在未来十年产生大量关于这些行星的新数据。最近，人们清楚地认识到，在许多这类行星的弱电离大气中，磁感应形式的新物理学起着重要作用，当温度超过1300 K左右时，会导致大气风的磁阻力和深度的欧姆耗散（这导致这些行星的半径膨胀）。**结合分析考虑和数值模型，我们将把这类系外行星的研究计划集中在三个特定主题上，旨在进一步测试和巩固磁感应情景。我们将阐明已知行星总群的磁阻和欧姆耗散随行星温度的预期趋势，鉴于大气电离与温度的指数依赖性，这应该是明显的。对于具体的、有很好特征的行星，我们还将探索几个直接受磁感应影响的可观测行星之间的相互作用，比如拖拽的风与半径膨胀的程度。最后，我们将研究高温日面电流的MHD失控辐合在这些区域局部增加加热的可能作用，这一过程可能是在几个热木星上观测推断的热反转的起源。**在低质量m矮星的宜居带中运行的类地系外行星是天文学搜索的焦点，因为一些偏差有利于它们的探测和表征。因此，它们可能会在未来十年为附近可居住世界的大气特征提供第一个目标。研究这些星球丰富的气候动力学是很重要的，它可以指导观测工作，并在获得数据时提供对数据的解释。**考虑到气候系统固有的复杂性，我们首选的方法是使用适应的气候模型来评估这些具有永久白天和黑夜的遥远星球的表面状况。我们的工作使用了一个先进的，但多功能的地球系统气候模拟器，具有完整的水文循环，可调节的陆地/海洋部分和诊断云处方，来研究m矮星周围可居住世界的气候。**我们在这类行星上的工作重点将是了解它们不寻常的水文循环，它们的大部分地表水可能以夜侧冰的形式被捕获（水被捕获的气候结构）。在这些行星的向阳面留下的残余水的数量将是决定这些世界是否适合居住的一个重要因素。更一般地说，我们的工作将探索这些行星的各种预期轨道配置（自旋速率、离心率、倾角、不同步）及其对气候的影响。在整个探索过程中，我们将努力量化地表条件和可居住性在多大程度上可以从一组天文测量中推断出来，这些测量将可用来描述这类行星的特征。

In recent years, the study of exoplanets has shifted from their detection to the characterization of their physical properties, in particular their atmospheres. The work on exoplanets proposed here consists of using some of the basic tools and principles of atmospheric science, originally developed for Earth, to deepen our understanding of the diversity of atmospheric behaviors expected on exoplanets and to clarify how those might shape astronomical observables. **1) Hot Exoplanets**Hot Jupiters/Saturns/Neptunes form the best characterized class of exoplanets and the next generation of astronomical observatories and surveys, such as TESS, JWST and ECho, will yield a wealth of new data on these planets over the next decade. Recently, it became clear that new physics in the form of magnetic induction plays an important role in the weakly-ionized atmospheres of many such planets, when temperatures exceed about 1300 K, leading to magnetic drag on the atmospheric winds and ohmic dissipation at depth (which results in radius inflation for these planets).**Using a combination of analytical considerations and numerical models, we will focus our research program for this class of exoplanets on three specific topics designed to further test and consolidate the magnetic induction scenario. We will clarify expected trends for magnetic drag and ohmic dissipation with planet temperature for the ensemble population of known planets, which should be manifest given the exponential dependence of atmospheric ionization with temperature. For specific, well characterized planets, we will also explore the interplay between several planetary observables which are directly impacted by magnetic induction, such as dragged winds vs. the degree of radius inflation. Finally, we will study the possible role of an MHD runaway convergence of electric currents on the hot dayside to locally increase heating in those regions, a process which could be at the origin of the thermal inversions observationally inferred on several hot Jupiters.**2) Habitable worlds around M-dwarfs**Terrestrial exoplanets orbiting in the habitable zone of low-mass M-dwarf stars are the focus of astronomical searches because several biases favor their detection and characterization. They may thus offer the first targets for atmospheric characterization of nearby habitable worlds in the next decade. Studying the rich climate dynamics of such worlds is important, to guide observational efforts and to provide an interpretation of the data when it becomes available.**The use of adapted climate models to evaluate surface conditions on such remote worlds with permanent day and night sides is our preferred approach, given the inherent complexity of the climate system. Our work uses an advanced, yet versatile, Earth-system climate simulator, with a full hydrological cycle, adjustable land/ocean fractions and diagnostic cloud prescriptions, to study the climates of habitable worlds around M-dwarfs. **A focus of our work on this class of planets will be to understand their unusual hydrological cycle, with the likely trapping of the majority of their surface water in the form of nightside ice (water-trapped climate configuration). The amount of residual water left on the dayside of such planets will be an important factor determining the habitability of these worlds. More generally, our work will explore a variety of expected orbital configurations for such planets (spin rate, eccentricity, obliquity, asynchronism) and their consequences for the climate. Throughout this exploration, we will strive to quantify to what extent surface conditions and habitability can be inferred from the set of astronomical measurements that will be available to characterize such planets.