Title: Detecting Surface Liquid Water on Exoplanets Show Chinese title

Title: 检测系外行星表面液态水

Abstract: Planets with large bodies of water on their surface will have more temperate and stable climates, and such planets are the ideal places for life-as-we-know-it to arise and evolve. A key science case for the Habitable Worlds Observatory (HWO) is to determine which planets host surface liquid water. Aside from its implications for planetary climate and astrobiology, detecting surface water on terrestrial exoplanets would place important constraints on our theories of planet formation and volatile delivery. Rotational variability in the reflectance of an exoplanet may reveal surface features rotating in and out of view, including oceans. Orbital changes in reflectance and polarization, meanwhile, are sensitive to the scattering phase function of the planetary surface, including specular reflection from large bodies of water. Although these techniques are applicable to all temperate terrestrial exoplanets, we focus in this document on the directly-imaged planets that are more likely to drive the HWO coronagraph design. Identification of water oceans relies on detecting a liquid, and using other lines of evidence to narrow that liquid down to being water. Liquids have smoother surfaces than most solids, and hence exhibit specular reflection instead of diffuse reflection. In practice, this makes lakes and oceans look dark from most illumination angles, but mirror-like at glancing angles. HWO is uniquely capable of identifying surface liquid oceans via their optical properties. Given that discovering an ocean on an exoplanet would confirm its status as a habitable world, this science case is literally the raison d'etre of the Habitable Worlds Observatory. Show Chinese abstract

Abstract: 有大量水体的行星将拥有更温和且稳定的气候，这样的行星是已知生命形式出现和进化的理想场所。宜居世界观测台（HWO）的一个关键科学目标是确定哪些行星拥有地表液态水。除了对行星气候和天体生物学的意义外，在类地系外行星上检测到地表水将对我们关于行星形成和挥发物输送的理论施加重要的限制。系外行星反射率的旋转变化可能揭示出随视图旋转进出的表面特征，包括海洋。同时，反射率和偏振的轨道变化对行星表面的散射相函数敏感，包括来自大面积水体的镜面反射。尽管这些技术适用于所有温带类地系外行星，但本文档重点关注那些更可能推动HWO日冕仪设计的直接成像行星。识别海洋依赖于检测液体，并利用其他证据将该液体缩小到是水。液体比大多数固体具有更平滑的表面，因此表现出镜面反射而非漫反射。实际上，这使得湖泊和海洋从大多数照明角度看起来很暗，但在倾斜角度下则像镜子一样。HWO能够通过其光学特性唯一地识别地表液态海洋。鉴于在系外行星上发现海洋将证实其作为宜居世界的资格，这个科学案例实际上是宜居世界观测台存在的根本原因。