Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets
Some inconvenient truths about biosignatures involving two chemical species on Earth-like exoplanets
复制标题
DOI：
10.1073/pnas.1401816111
复制
发表时间：
2014-04
期刊：
影响因子：
--
通讯作者：
H. Rein;Y. Fujii;D. Spiegel
中科院分区：
文献类型：
--
作者：
H. Rein;Y. Fujii;D. Spiegel
意义 在太阳系外的行星上寻找生命是人类有史以来最引人注目的探索之一。在此类行星上寻找生命迹象的一种经常建议的方法是寻找强烈化学不平衡的光谱特征。本文介绍了与此方法相关的一个重要的潜在混乱来源。任何系外行星都可能拥有污染行星光谱的卫星。一般来说，我们无法排除月球的存在。通过计算原则上对类地行星可获得的最乐观的光谱分辨率，我们表明在可预见的未来推断系外行星上的生物圈可能超出了我们的能力范围。在太阳系外行星大气层中检测到强烈的热化学不平衡被认为是潜在的生物特征。在本文中，我们提出了一种先前未识别的假阳性，它可以模拟不平衡或涉及两种化学物质的任何其他生物特征。我们考虑这样一种情况：系外行星拥有一颗卫星，它有自己的大气层，并且两个大气层都不处于化学不平衡状态。我们的结果表明，行星和月球的综合光谱与处于强化学不平衡状态的单个物体的综合光谱非常相似。我们得出了直接成像行星和凌日行星可以获得的最大光谱分辨率的严格限制。即使是未来几十年可能实现的理想化天基光谱仪的光谱分辨率通常也不足以打破简并性。只有在能够明确识别两种化学物质的吸收特征并且它们的组合深度超过 100% 的情况下，才能在同一物体中明确确认这两种化学物质。
Significance The search for life on planets outside our own solar system is among the most compelling quests that humanity has ever undertaken. An often suggested method of searching for signs of life on such planets involves looking for spectral signatures of strong chemical disequilibrium. This article introduces an important potential source of confusion associated with this method. Any exoplanet can host a moon that contaminates the planetary spectrum. In general, we will be unable to exclude the existence of a moon. By calculating the most optimistic spectral resolution in principle obtainable for Earth-like planets, we show that inferring a biosphere on an exoplanet might be beyond our reach in the foreseeable future. The detection of strong thermochemical disequilibrium in the atmosphere of an extrasolar planet is thought to be a potential biosignature. In this article we present a previously unidentified kind of false positive that can mimic a disequilibrium or any other biosignature that involves two chemical species. We consider a scenario where the exoplanet hosts a moon that has its own atmosphere and neither of the atmospheres is in chemical disequilibrium. Our results show that the integrated spectrum of the planet and the moon closely resembles that of a single object in strong chemical disequilibrium. We derive a firm limit on the maximum spectral resolution that can be obtained for both directly imaged and transiting planets. The spectral resolution of even idealized space-based spectrographs that might be achievable in the next several decades is in general insufficient to break the degeneracy. Both chemical species can only be definitively confirmed in the same object if absorption features of both chemicals can be unambiguously identified and their combined depth exceeds 100%.