Call for a framework for reporting evidence for life beyond Earth

Green, James; Hoehler, Tori M.; Neveu, Marc; Domagal-Goldman, Shawn; Scalice, D.; Voytek, Mary A.

doi:10.1038/s41586-021-03804-9

Cited by 43 publications

(36 citation statements)

References 17 publications

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“…One of the most important open questions regarding the ultimate goal of detecting extrasolar life will require to put our results in context with life detection frameworks (e.g. Green et al 2021;. Our ongoing retrieval efforts could be useful for the fine-tuning of such frameworks.…”

Section: Next Stepsmentioning

confidence: 99%

Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs

Alei,

Konrad,

Angerhausen

et al. 2022

Preprint

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Context. An important future goal in exoplanetology is to detect and characterize potentially habitable planets. Concepts for future space missions have already been proposed: from a large UV-Optical-Infrared space mission for studies in reflected light, to the Large Interferometer for Exoplanets (LIFE) for analyzing the thermal portion of the planetary spectrum. Using nulling interferometry, LIFE will allow us to constrain the radius and effective temperature of (terrestrial) exoplanets, as well as provide unique information about their atmospheric structure and composition.Aims. We explore the potential of LIFE in characterizing emission spectra of Earth at various stages of its evolution. This allows us (1) to test the robustness of Bayesian atmospheric retrieval frameworks when branching out from a Modern Earth scenario while still remaining in the realm of habitable (and inhabited) exoplanets, and (2) to refine the science requirements for LIFE for the detection and characterization of habitable, terrestrial exoplanets. Methods. We perform Bayesian retrievals on simulated spectra of 8 different scenarios, which correspond to cloud-free and cloudy spectra of four different epochs of the evolution of the Earth. Assuming a distance of 10 pc and a Sun-like host star, we simulate observations obtained with LIFE using its simulator LIFEsim, considering all major astrophysical noise sources.Results. With the nominal spectral resolution (R = 50) and signal-to-noise ratio (assumed to be S/N = 10 at 11.2 µm), we can identify the main spectral features of all the analyzed scenarios (most notably CO 2 , H 2 O, O 3 , CH 4 ). This allows us to distinguish between inhabited and lifeless scenarios. Results suggest that particularly O 3 and CH 4 yield an improved abundance estimate by doubling the S/N from 10 to 20. Neglecting clouds in the retrieval still allows for a correct characterization of the atmospheric composition. However, correct cloud modeling is necessary to avoid biases in the retrieval of the correct thermal structure. Conclusions. From this analysis, we conclude that the baseline requirements for R and S/N are sufficient for LIFE to detect O 3 and CH 4 in the atmosphere of an Earth-like planet with an abundance of O 2 of around 2% in volume mixing ratio. Doubling the S/N would allow a clearer detection of these species at lower abundances. This information is relevant in terms of the LIFE mission planning. We also conclude that cloud-free retrievals of cloudy planets can be used to characterize the atmospheric composition of terrestrial habitable planets, but not the thermal structure of the atmosphere. From the inter-model comparison performed, we deduce that differences in the opacity tables (caused by e.g. a different line wing treatment) may be an important source of systematic errors.

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Section: Next Stepsmentioning

confidence: 99%

Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs

Alei,

Konrad,

Angerhausen

et al. 2022

Preprint

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“…The NASA-sponsored “NfoLD/NExSS Standards of Evidence for Life Detection Community Workshop” laid out a general framework for rigorous biosignature assessment: (1) the detection of a signal; (2) the identification of that signal; (3) an assessment of abiotic sources for that signal; (4) an assessment of biological sources for that signal; and (5) independent lines of evidence to support the hypothesis. Steps 1 and 2 deal with the detection of a signal of interest, while steps 3–5 serve to interpret that signal in the context of its environment, our understanding of life, and subsequent data [ 74 , 75 ].…”

Section: Discussionmentioning

confidence: 99%

Searching for Life, Mindful of Lyfe’s Possibilities

Wong

Bartlett

Chen

et al. 2022

Life

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We are embarking on a new age of astrobiology, one in which numerous interplanetary missions and telescopes will be designed, built, and launched with the explicit goal of finding evidence for life beyond Earth. Such a profound aim warrants caution and responsibility when interpreting and disseminating results. Scientists must take care not to overstate (or over-imply) confidence in life detection when evidence is lacking, or only incremental advances have been made. Recently, there has been a call for the community to create standards of evidence for the detection and reporting of biosignatures. In this perspective, we wish to highlight a critical but often understated element to the discussion of biosignatures: Life detection studies are deeply entwined with and rely upon our (often preconceived) notions of what life is, the origins of life, and habitability. Where biosignatures are concerned, these three highly related questions are frequently relegated to a low priority, assumed to be already solved or irrelevant to the question of life detection. Therefore, our aim is to bring to the fore how these other major astrobiological frontiers are central to searching for life elsewhere and encourage astrobiologists to embrace the reality that all of these science questions are interrelated and must be furthered together rather than separately. Finally, in an effort to be more inclusive of life as we do not know it, we propose tentative criteria for a more general and expansive characterization of habitability that we call genesity.

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“…For instance, the biosignature search community is developing many mature frameworks for designing and interpreting the results of life detection experiments, including the development and selection of good biosignatures to search for, selection criteria and prioritization of target lists, and frameworks for determining the confidence of detection and dealing with ambiguity (Stark et al 2019;Truitt et al 2020;Tuchow & Wright 2020, 2021Neveu et al 2018;Green et al 2021, to give a few recent examples in addition to the references in Section 1).…”

Section: Discussionmentioning

confidence: 99%

The Case for Technosignatures: Why They May Be Abundant, Long-lived, Highly Detectable, and Unambiguous

Wright,

Haqq-Misra,

Frank

et al. 2022

Preprint

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The intuition suggested by the Drake Equation implies that technology should be less prevalent than biology in the galaxy. However, it has been appreciated for decades in the SETI community that technosignatures could be more abundant, longer-lived, more detectable, and less ambiguous than biosignatures.We collect the arguments for and against technosignatures' ubiquity, and discuss the implications of some properties of technological life that fundamentally differ from non-technological life, in the context of modern astrobiology: it can spread among the stars to many sites, it can be more easily detected at large distances, and it can produce signs that are unambiguously technological.As an illustration in terms of the Drake Equation, we consider two Drake-like equations, for technosignatures (calculating N (tech)) and biosignatures (calculating N (bio)). We argue that Earth and humanity may be poor guides to the longevity term L, and that its maximum value could be very large, in that technology can outlive its creators and even its host star. We conclude that while the Drake equation implies that N (bio) N (tech), it is also plausible that N (tech) N (bio). As a consequence, as we seek possible indicators of extraterrestrial life, for instance via characterization of the atmospheres of habitable exoplanets, we should search for both biosignatures and technosignatures. This exercise also illustrates ways in which biosignature and technosignature searches can complement and supplement each other, and how methods of technosignature search, including old ideas from SETI, can inform the search for biosignatures and life generally.

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Call for a framework for reporting evidence for life beyond Earth

Cited by 43 publications

References 17 publications

Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs

Large Interferometer For Exoplanets (LIFE): V. Diagnostic potential of a mid-infrared space-interferometer for studying Earth analogs

Searching for Life, Mindful of Lyfe’s Possibilities

The Case for Technosignatures: Why They May Be Abundant, Long-lived, Highly Detectable, and Unambiguous

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