Carbon-bearing Molecules in a Possible Hycean Atmosphere
Carbon-bearing Molecules in a Possible Hycean Atmosphere
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DOI：
10.3847/2041-8213/acf577
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发表时间：
2023-09
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通讯作者：
N. Madhusudhan;Subhajit Sarkar;Savvas Constantinou;Måns Holmberg;Anjali A. A. Piette-Anjali-A.-A.-Piette-49809422;Julianne I. Moses
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作者：
N. Madhusudhan;Subhajit Sarkar;Savvas Constantinou;Måns Holmberg;Anjali A. A. Piette-Anjali-A.-A.-Piette-49809422;Julianne I. Moses
在系外行星大气中寻找宜居环境和生物标志物是系外行星科学的圣杯。可居住的类地系外行星的大气特征的探测具有挑战性，因为它们的行星-恒星大小对比度小，大气稀薄，平均分子量高。最近，一类新的宜居系外行星被提出，称为Hycean World，定义为具有富H2大气的温带海洋覆盖的世界。与相同质量的岩石行星相比，它们的大小和延伸的大气层使海王星世界更容易被JWST的大气光谱探测到。在这里，我们报告了用JWST NIRISS和NIRSpec仪器在0.95.2μm范围内观测到的候选Hycean世界K2-18b的透射谱。光谱显示，甲烷(CH4)和二氧化碳(CO2)分别在5σ和3σ置信度下有很强的探测，在富氢大气中，∼的体积混合比分别为1%。丰富的CH4和CO2，以及没有检测到氨(NH3)，与K2-18b上温和的富氢大气下存在海洋的化学预测一致。光谱还表明，二甲基硫化物(DMS)的潜在迹象，已被预测为海王星世界中一个可观察到的生物标志物，促使人们考虑这颗行星上可能的生物活动。CH4的探测解决了温带系外行星长期缺少甲烷的问题，以及以前观测到的K2-18b大气成分的简并性。我们讨论了这些发现的可能影响，开放的问题，以及未来的观察，以探索这种在其他地方寻找生命的新制度。
The search for habitable environments and biomarkers in exoplanetary atmospheres is the holy grail of exoplanet science. The detection of atmospheric signatures of habitable Earth-like exoplanets is challenging owing to their small planet–star size contrast and thin atmospheres with high mean molecular weight. Recently, a new class of habitable exoplanets, called Hycean worlds, has been proposed, defined as temperate ocean-covered worlds with H2-rich atmospheres. Their large sizes and extended atmospheres, compared to rocky planets of the same mass, make Hycean worlds significantly more accessible to atmospheric spectroscopy with JWST. Here we report a transmission spectrum of the candidate Hycean world K2-18 b, observed with the JWST NIRISS and NIRSpec instruments in the 0.9–5.2 μm range. The spectrum reveals strong detections of methane (CH4) and carbon dioxide (CO2) at 5σ and 3σ confidence, respectively, with high volume mixing ratios of ∼1% each in a H2-rich atmosphere. The abundant CH4 and CO2, along with the nondetection of ammonia (NH3), are consistent with chemical predictions for an ocean under a temperate H2-rich atmosphere on K2-18 b. The spectrum also suggests potential signs of dimethyl sulfide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet. The detection of CH4 resolves the long-standing missing methane problem for temperate exoplanets and the degeneracy in the atmospheric composition of K2-18 b from previous observations. We discuss possible implications of the findings, open questions, and future observations to explore this new regime in the search for life elsewhere.