[论文解读] The runaway greenhouse radius inflation effect - An observational diagnostic to probe water on Earth-size planets and test the Habitable Zone concept

Martin Turbet, D. Ehrenreich+3 more|arXiv (Cornell University)|Jun 8, 2019

Stellar, planetary, and galactic studies参考文献 64被引用 16

一句话总结

本文提出，处于后失控温室状态的类地系系外行星可能因厚重、炽热的水汽主导大气而表现出显著的大气半径膨胀——可达500公里或以上。该效应可通过凌星测光法与径向速度测量探测，为检验宜居带内缘及实证约束温带系外行星上失控温室转变阈值与水丰度提供一种新颖的观测诊断手段。

ABSTRACT

Planets similar to Earth - but slightly more irradiated - are expected to enter into a runaway greenhouse state, where all surface water rapidly evaporates, forming an optically thick H2O-dominated atmosphere. For Earth, this extreme climate transition is thought to occur for a ~6% increase only of the solar luminosity, though the exact limit at which the transition would occur is still a highly debated topic. In general, the runaway greenhouse is believed to be a fundamental process in the evolution of Earth-size, temperate planets. Using 1-D radiative-convective climate calculations accounting for thick, hot water vapour-dominated atmospheres, we evaluate the transit atmospheric thickness of a post-runaway greenhouse atmosphere, and find that it could possibly reach over a thousand kilometers (i.e., a few tens of % of Earth radius). This abrupt radius inflation - resulting from the runaway-greenhouse-induced transition - could be detected statistically by ongoing and upcoming space missions such as TESS, CHEOPS and PLATO (combined with precise radial velocity mass measurements with ground-based spectrographs such as ESPRESSO, CARMENES or SPIRou), or even in particular cases in multiplanetary systems such as TRAPPIST-1 (when masses and radii will be known with good enough precision). This result provides the community with an observational test of (1) the concept of runaway greenhouse, that defines the inner edge of the traditional Habitable Zone, and the exact limit of the runaway greenhouse transition. In particular, this could provide an empirical measurement of the irradiation at which Earth analogs transition from a temperate to a runaway greenhouse climate state. This astronomical measurement would make it possible to statistically estimate how close Earth is from the runaway greenhouse. (2) the presence (and statistical abundance) of water in temperate, Earth-size exoplanets.

研究动机与目标

• 开发失控温室转变的观测检验方法，这是定义宜居带内缘的关键过程。
• 评估在后失控温室状态下，类地系行星的大气半径膨胀效应是否可被当前及未来空间任务探测到。
• 利用半径膨胀的存在与否作为探测温带类地系系外行星中水含量与大气逃逸效率的探针。
• 通过实证方法估算类地行星在类太阳恒星周围发生失控温室转变的辐照水平，特别是针对类太阳恒星。
• 通过分析行星群体中半径膨胀的幅度，约束温带系外行星中水的统计丰度。

提出的方法

• 利用一维辐射对流气候模型（LMD通用反演模型）模拟不同恒星辐照条件下的大气热力结构。
• 将表观大气厚度定义为湿对流层顶部（云底）的高度，即光学厚水汽大气的顶部。
• 比较温带类地气候与完全蒸发海洋的后失控温室状态下的行星半径膨胀。
• 模拟不同H2O连续谱数据库（BPS与MT_CKD）对计算温度与大气标高影响。
• 利用能量限制逃逸近似估算半径膨胀增强导致的大气逃逸速率增加。
• 提出多行星系统（如TRAPPIST-1）以及TESS、CHEOPS与PLATO等任务可通过精确的半径与质量测量探测该效应。

实验结果

研究问题

• RQ1 后失控温室状态下的半径膨胀效应是否可在凌星系外行星中被观测到？
• RQ2 在经历失控温室转变的类地系行星中，大气半径膨胀的幅度有多大？
• RQ3 如何利用该效应的探测结果实证确定失控温室转变的辐照阈值？
• RQ4 该半径膨胀信号在多大程度上可约束温带系外行星的水含量与大气逃逸效率？
• RQ5 若未观测到半径膨胀，对行星群体中水含量或逃逸效率意味着什么？

主要发现

• 后失控温室状态可使类地系行星的表观大气厚度膨胀至500公里以上，极端情况下可达1,000公里。
• 半径膨胀效应导致行星半径增加达百分之几十，具体取决于大气质量和温度结构。
• 模型预测后失控温室状态下的地表温度约为1,700 K，略低于以往估计，原因在于H2O连续谱数据库的差异。
• 该效应原则上可被TESS、CHEOPS与PLATO探测到，尤其在具有精确质量与半径测量的多行星系统中。
• 强烈的半径膨胀信号表明行星保留了大量水，而其缺失则限制了最大水含量或暗示了高效的的大气逃逸。
• 该效应或可实现对类地行星在类太阳恒星周围发生失控温室转变的辐照水平的实证测量，尤其针对类太阳恒星。