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描述它们的物理性质，特别是它们的大气层。这里提出的关于系外行星的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，并澄清这些行为可能如何形成天文观测数据。 1)热系外行星 热木星/土星/海王星形成了特征最好的一类系外行星，下一代天文观测站和观测，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，人们清楚地看到，磁感应形式的新物理在许多此类行星的弱电离大气中发挥了重要作用，当温度超过约1300K时，会导致大气层风的磁阻力和深层的欧姆耗散(这会导致这些行星的半径膨胀)。 利用分析考虑和数值模型的结合，我们将把我们对这类系外行星的研究计划集中在三个特定的主题上，旨在进一步测试和巩固磁感应情景。我们将阐明已知行星群的磁阻力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相关性，这一趋势应该是明显的。对于特定的、具有良好特征的行星，我们还将探索几个直接受磁感应影响的行星观测参数之间的相互作用，例如拖曳风与半径膨胀程度。最后，我们将研究在炎热的白天，MHD失控的电流会聚对那些地区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到的热逆的起源。 2)M-矮星周围的宜居世界 在低质量M-矮星的宜居带内运行的类地系外行星是天文搜索的重点，因为有几个偏向有利于它们的探测和表征。因此，它们可能为未来十年附近宜居世界的大气特征提供第一个目标。研究这些世界丰富的气候动态对于指导观测工作并在数据可用时提供解释是重要的。 考虑到气候系统的内在复杂性，使用经过调整的气候模型来评估这些具有永久性昼夜边界的偏远世界的表面状况是我们首选的方法。我们的工作使用了一种先进而通用的地球系统气候模拟器，它具有完整的水文循环、可调的陆地/海洋比例和诊断性云处方，以研究M-矮星周围宜居世界的气候。 我们在这类行星上的工作重点将是了解它们不同寻常的水文循环，可能以夜间冰(水困气候配置)的形式捕获它们的大部分地表水。这些行星白天剩余的水量将是决定这些星球是否宜居的一个重要因素。更广泛地说，我们的工作将探索这类行星的各种预期轨道构型(自转速度、偏心率、倾斜度、异步性)及其对气候的影响。在整个探索过程中，我们将努力量化从一组天文测量中可以推断出这些行星的表面状况和宜居性的程度，这些测量将可用于描述这些行星的特征。

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.