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%.With almost a thousand confirmed exoplanets [Open Exoplanet Catalogue (
1)], the prospects of detecting signs of a biosphere on a body outside our own solar system are more promising than ever before. However, there are still huge technological and theoretical challenges to overcome before one can hope to make a clear detection of life on an exoplanet. In this article, we discuss one of these complications, the possibility of false positives due to the presence of an exomoon orbiting the exoplanet.There are many ways that life on an exoplanet might affect the planet’s appearance, ranging from deliberate signals from intelligent civilizations (
2) to subtler signs of simple life. To characterize an extrasolar world as fully as possible, we ideally would measure its spectrum as a function of time in both the optical and the infrared parts of the spectrum (e.g., refs.
3–
6). For example, spectral evidence of water could suggest that a planet might be habitable. It has also been suggested that an intriguing indication of life might be an increase in the planet’s albedo toward the infrared part of the spectrum, which on Earth can be associated with vegetation (
7). However, these features alone would not be smoking-gun proof of the presence of life. The terms “biomarker” and “biosignature” generally refer to chemicals or combinations of chemicals that could be produced by life and that could not be (or are unlikely to be) produced abiotically; hereafter, we use these terms interchangeably. If biosignature gases are detected in the spectrum of an exoplanet, the probability that they actually indicate life depends both on the prior probability of life (
8) and on the probability that the observed spectroscopic feature could be produced abiotically. The latter possibility is the subject of this paper.Byproducts of metabolism are often thought of as the most promising biomarker (
9–
15). More specifically, an extreme thermodynamic disequilibrium of two molecules in the atmosphere is considered a biosignature (
16–
18). An example of two such species is the simultaneous presence of O
2 and a reduced gas such as CH
4. It is important to point out that a disequilibrium in a planet’s atmosphere should not be considered as clear evidence for life. [Also note that the Earth might have never had a phase of strong, observable O
2/CH
4 disequilibrium (
19).] There is a long list of abiotic sources that could also create a disequilibrium such as impacts (
20), photochemistry (
21), and geochemistry (
14).In this article, we describe a previously unidentified scenario for a possible false positive biosignature. If the exoplanet hosts a moon that has an atmosphere itself, the simultaneous observation of the planet and moon modifies the observed spectrum (see also refs.
22 and
23) and can produce a signal that looks like a disequilibrium in one atmosphere but is in fact created by two atmospheres blended together. It might be extremely difficult to discern that an exoplanet even has a moon, let alone that one component of a two-chemical biosignature comes from the moon instead of the planet.The outline of this article is as follows. We first describe our model atmospheres and present simulated spectra. Using those synthetic spectra, we show that the combined spectrum from an oxygen-rich atmosphere such as that of the Earth and a methane-rich atmosphere such as that of Titan indeed looks like it could have come from a single atmosphere with a strong disequilibrium. We then calculate a strong upper limit on the spectral resolution of such a system as observed from Earth under ideal conditions with a plausibly sized space telescope. Our estimate shows that the spectral resolution for such a system is unlikely to exceed ∼1,600 with foreseeable technology. Given this maximum possible resolution, discriminating between a single planet and a planet–moon system is in general unlikely to be possible.
† Nevertheless, we conclude with a summary and a positive outlook with two possibilities that can provide genuine biosignatures. The first possibility is to find a single chemical species that is sufficient to indicate life. The second one requires the unambiguous identification of both species’ absorption features and the combined depth of the features needs to exceed 100%.
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