TRAPPIST-1 is a fantastic nearby (∼39.14 light years) planetary system made of at least seven transiting terrestrial-size, terrestrial-mass planets all receiving a moderate amount of irradiation. To date, this is the most observationally favourable system of potentially habitable planets known to exist. Since the announcement of the discovery of the TRAPPIST-1 planetary system in 2016, a growing number of techniques and approaches have been used and proposed to characterize its true nature. Here we have compiled a state-of-the-art overview of all the observational and theoretical constraints that have been obtained so far using these techniques and approaches. The goal is to get a better understanding of whether or not TRAPPIST-1 planets can have atmospheres, and if so, what they are made of. For this, we surveyed the literature on TRAPPIST-1 about topics as broad as irradiation environment, planet formation and migration, orbital stability, effects of tides and Transit Timing Variations, transit observations, stellar contamination, density measurements, and numerical climate and escape models. Each of these topics adds a brick to our understanding of the likely—or on the contrary unlikely—atmospheres of the seven known planets of the system. We show that (i) Hubble Space Telescope transit observations, (ii) bulk density measurements comparison with H2-rich planets mass-radius relationships, (iii) atmospheric escape modelling, and (iv) gas accretion modelling altogether offer solid evidence against the presence of hydrogen-dominated—cloud-free and cloudy—atmospheres around TRAPPIST-1 planets. This means that the planets are likely to have either (i) a high molecular weight atmosphere or (ii) no atmosphere at all. There are several key challenges ahead to characterize the bulk composition(s) of the atmospheres (if present) of TRAPPIST-1 planets. The main one so far is characterizing and correcting for the effects of stellar contamination. Fortunately, a new wave of observations with the James Webb Space Telescope and near-infrared high-resolution ground-based spectrographs on existing very large and forthcoming extremely large telescopes will bring significant advances in the coming decade.

中文翻译：

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TRAPPIST-1 系统中可能存在的行星大气综述

TRAPPIST-1 是一个奇妙的邻近（~39.14 光年）行星系统，由至少七颗凌日类地大小、类地质量的行星组成，所有行星都接受适度的辐射。迄今为止，这是已知存在的最有利于观测的潜在宜居行星系统。自 2016 年宣布发现 TRAPPIST-1 行星系统以来，越来越多的技术和方法已被使用并提出来表征其真实性质。在这里，我们编制了迄今为止使用这些技术和方法获得的所有观察和理论约束的最新概述。目标是更好地了解 TRAPPIST-1 行星是否有大气层，如果有，它们是由什么组成的。为了这，我们调查了有关 TRAPPIST-1 的文献，涉及的主题广泛，如辐照环境、行星形成和迁移、轨道稳定性、潮汐和过境时间变化的影响、过境观测、恒星污染、密度测量以及数值气候和逃逸模型。这些主题中的每一个都为我们对系统中七颗已知行星的可能——或者相反不太可能——的大气层的理解增加了一块砖。我们表明 (i) 哈勃太空望远镜凌日观测，(ii) 体积密度测量与富含 H2 的行星质量半径关系的比较，(iii) 大气逃逸模型和 (iv) 气体吸积模型共同提供了反对存在的确凿证据TRAPPIST-1 行星周围以氢为主的无云和多云大气。这意味着这些行星可能具有（i）高分子量大气或（ii）根本没有大气。要表征 TRAPPIST-1 行星大气（如果存在）的整体成分，还面临几个关键挑战。到目前为止，主要的任务是表征和纠正恒星污染的影响。幸运的是，詹姆斯韦伯太空望远镜的新一波观测以及在现有超大型和即将推出的超大型望远镜上使用近红外高分辨率地面光谱仪进行的新一波观测将在未来十年带来重大进展。到目前为止，主要的任务是表征和纠正恒星污染的影响。幸运的是，詹姆斯韦伯太空望远镜的新一波观测以及在现有超大型和即将推出的超大型望远镜上使用近红外高分辨率地面光谱仪进行的新一波观测将在未来十年带来重大进展。到目前为止，主要的任务是表征和纠正恒星污染的影响。幸运的是，詹姆斯韦伯太空望远镜的新一波观测以及在现有超大型和即将推出的超大型望远镜上使用近红外高分辨率地面光谱仪进行的新一波观测将在未来十年带来重大进展。