Recent analysis of the planet K2-18 b has shown the presence of water vapour in its atmosphere. While the H₂O detection is significant, the Hubble Space Telescope (HST) WFC3 spectrum suggests three possible solutions of very different nature which can equally match the data. The three solutions are a primary cloudy atmosphere with traces of water vapour (cloudy sub-Neptune), a secondary atmosphere with a substantial amount (up to 50% Volume Mixing Ratio) of H₂O (icy/water world) and/or an undetectable gas such as N₂ (super-Earth). Additionally, the atmospheric pressure and the possible presence of a liquid/solid surface cannot be investigated with currently available observations. In this paper we used the best fit parameters from Tsiaras et al. (Nat. Astron. 3, 1086, 2019) to build James Webb Space Telescope (JWST) and Ariel simulations of the three scenarios. We have investigated 18 retrieval cases, which encompass the three scenarios and different observational strategies with the two observatories. Retrieval results show that twenty combined transits should be enough for the Ariel mission to disentangle the three scenarios, while JWST would require only two transits if combining NIRISS and NIRSpec data. This makes K2-18 b an ideal target for atmospheric follow-ups by both facilities and highlights the capabilities of the next generation of space-based infrared observatories to provide a complete picture of low mass planets.

最近对 K2-18 b 行星的分析表明，其大气中存在水蒸气。虽然 H ₂ O 检测意义重大，但哈勃太空望远镜 (HST) WFC3 光谱提出了三种性质截然不同的可能解决方案，它们可以同样匹配数据。这三种解决方案是具有微量水蒸气的主要多云大气（多云次海王星），具有大量（高达 50% 体积混合比）H ₂ O（冰冷/水世界）的次要大气和/或N ₂等不可检测的气体（超级地球）。此外，目前可用的观测无法研究大气压力和液体/固体表面的可能存在。在本文中，我们使用了 Tsiaras 等人的最佳拟合参数。（自然。Astron。3, 1086, 2019) 建立詹姆斯韦伯太空望远镜 (JWST) 和爱丽儿模拟三种场景。我们调查了 18 个检索案例，包括三个场景和两个天文台的不同观测策略。检索结果表明，如果结合 NIRISS 和 NIRSpec 数据，20 次组合凌日应该足以让 Ariel 任务解开这三种情况，而 JWST 只需要两次凌日。这使得 K2-18 b 成为两个设施进行大气跟踪的理想目标，并突出了下一代天基红外天文台提供低质量行星完整图片的能力。