The nature of exoplanet K2-18 b is under intense debate. A recent theory, purportedly supported by JWST data (Astrophys. J. Lett. 956, L13; 2023), suggested that this 2.6-R_⊕ planet in the classical ‘habitable zone’ of its star has a thin hydrogen-rich atmosphere but a water-rich interior and a global liquid water ocean — a ‘Hycean’ world — boosting its habitability prospects. Nicholas Wogan and colleagues temper these expectations by testing the feasibility of this scenario with coupled 1D photochemical and climate models.

The authors simulate three states for K2-18 b: a lifeless Hycean world, an inhabited one with a simple biosphere that affects the atmospheric chemistry, and a gas-rich planet with a deep atmosphere akin to standard gas or icy giants. The results show that a lifeless Hycean world cannot explain the observed methane bands in the JWST spectra. Instead, the other two scenarios can fit the data with comparable goodness. Wogan et al. then compare the complexity of both possibilities and show that a cool habitable surface on K2-18 b would require very ad-hoc conditions to preserve a ~1-bar H₂ atmosphere from stellar radiation and avoid a runaway greenhouse effect at the same time. On the other hand, a standard ‘mini-Neptune’ can explain JWST observations straightforwardly.

系外行星 K2-18 b 的性质引起了激烈的争论。最近的一项理论，据称得到了 JWST 数据的支持（Astrophys. J. Lett. 956，L13；2023），表明这颗 2.6- R _⊕行星位于其恒星的经典“宜居带”，拥有稀薄的富氢大气层，但富含水的内陆和全球液态水海洋——“Hycean”世界——提高了其宜居性前景。Nicholas Wogan 及其同事通过结合一维光化学和气候模型来测试这种情况的可行性，从而调整了这些预期。

作者模拟了 K2-18 b 的三种状态：一个没有生命的海西亚世界，一个有人居住的星球，其生物圈影响大气化学，以及一个富含气体的行星，其深层大气类似于标准气体或冰巨星。结果表明，没有生命的海西亚世界无法解释 JWST 光谱中观察到的甲烷带。相反，其他两种场景可以以相当的精度拟合数据。沃根等人。然后比较两种可能性的复杂性，并表明 K2-18 b 上凉爽的宜居表面需要非常特殊的条件才能保护约 1 巴的 H ₂大气免受恒星辐射，同时避免失控的温室效应。另一方面，标准的“迷你海王星”可以直接解释 JWST 的观测结果。