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Call for a framework for reporting evidence for life beyond Earth


Our generation could realistically be the one to discover evidence of life beyond Earth. With this privileged potential comes responsibility. The magnitude of the question of whether we are alone in the Universe, and the public interest therein, opens the possibility that results may be taken to imply more than the observations support, or than the observers intend. As life-detection objectives become increasingly prominent in space sciences, it is essential to open a community dialogue about how to convey information in a subject matter that is diverse, complicated and has a high potential to be sensationalized. Establishing best practices for communicating about life detection can serve to set reasonable expectations on the early stages of a hugely challenging endeavour, attach value to incremental steps along the path, and build public trust by making clear that false starts and dead ends are an expected and potentially productive part of the scientific process. Here we endeavour to motivate and seed the discussion with basic considerations and offer an example of how such considerations might be incorporated and applied in a proof-of-concept-level framework. Everything mentioned herein, including the name of the confidence scale, is intended not as a prescription, but simply as the beginning of an important dialogue.

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Fig. 1: The CoLD scale.


  1. Crowe, M. The Extraterrestrial Life Debate: Antiquity to 1915: A Source Book (Univ. Notre Dame Press, 2008).

  2. Dick, S. J. Life on Other Worlds: The 20th-Century Extraterrestrial Life Debate (Cambridge Univ. Press, 1998).

  3. Blastland, M. et al. Five rules for evidence communication. Nature 587, 362–364 (2020). A commentary on effective science communication.

    ADS  CAS  Article  Google Scholar 

  4. National Academies of Sciences, Engineering, and Medicine An Astrobiology Strategy for the Search for Life in the Universe (The National Academies Press, 2019).

  5. Neveu, M. et al. The ladder of life detection. Astrobiology 18, 1375–1402 (2018).

    ADS  Article  Google Scholar 

  6. Bean, J. L., Abbot, D. S. & Kempton, E. M.-R. A statistical comparative planetology approach to the hunt for habitable exoplanets and life beyond the Solar System. Astrophys. J. Lett. 841, L24 (2017).

    ADS  Article  Google Scholar 

  7. Benner, S. A. Defining life. Astrobiology 10, 1021–1030 (2010).

    ADS  Article  Google Scholar 

  8. Binzel, R. P. The Torino impact hazard scale. Planet. Space Sci. 48, 297–303 (2000).

    ADS  Article  Google Scholar 

  9. Morrison, D. et al. in Mitigation of Hazardous Comets and Asteroids (eds Belton, M. et al.) 252–390 (Cambridge Univ. Press, 2004). A description of the considerations that went into building and updating the Torino scale.

  10. Beauchamp, P. M. et al. Technology Readiness Levels: Best Practices Guide NASA SP-20205003605 (NASA, 2020);

  11. McKay, D. S. et al. Search for past life on Mars: possible relic biogenic activity in Martian meteorite ALH84001. Science 273, 924–930 (1996). The original report of evidence of life, cast in a binary light, in the Martian meteorite ALH84001.

    ADS  CAS  Article  Google Scholar 

  12. Romanek, C. S. et al. Record of fluid–rock interactions on Mars from the meteorite ALH84001. Nature 372, 655–657 (1994).

    ADS  CAS  Article  Google Scholar 

  13. Sawyer, K. The Rock from Mars: A Detective Story on Two Planets (Random House, 2006).

  14. Williford, K. H. et al. The NASA Mars 2020 Rover Mission and the Search for Extraterrestrial Life. From Habitability to Life on Mars 275–308 (Elsevier, 2018).

  15. Muirhead, B. K. et al. Mars sample return mission concept status. Acta Astronaut. 176, 131–138 (2020).

    ADS  Article  Google Scholar 

  16. Farmer, J. D. & Des Marais, D. J. Exploring for a record of ancient Martian life. J. Geophys. Res. 104, 26977–26995 (1999).

    ADS  Article  Google Scholar 

  17. Cady, S. L. et al. Morphological biosignatures and the search for life on Mars. Astrobiology 3, 351–368 (2003).

    ADS  CAS  Article  Google Scholar 

  18. Bosak, T. et al. Microbial sedimentology of stromatolites in Neoproterozoic cap carbonates. Paleontol. Soc. Pap. 19, 51–76 (2013).

    Article  Google Scholar 

  19. Summons, R. E. & Hallmann, C. in Treatise on Geochemistry 2nd edn (eds Falkowski, P. & Freeman, K.) 33–46 (Elsevier, 2014).

  20. Capezzuoli, E., Gandin, A. & Pedley, M. Decoding tufa and travertine (fresh-water carbonates) in the sedimentary record: the state of the art. Sedimentology 61, 1–21 (2014).

    Article  Google Scholar 

  21. Horita, J. Some perspectives on isotope biosignatures for early life. Chem. Geol. 218, 171–186 (2005).

    ADS  CAS  Article  Google Scholar 

  22. National Academies of Sciences, Engineering, and Medicine Decadal Survey on Astronomy and Astrophysics 2020 (The National Academies Press, 2021).

  23. Meadows, V. S. et al. Exoplanet biosignatures: understanding oxygen as a biosignature in the context of its environment. Astrobiology 18, 630–662 (2018).

  24. Checlair, J. H. et al. Probing the capability of future direct-imaging missions to spectrally constrain the frequency of Earth-like planets. Astron. J. 161, 150 (2021).

    ADS  Article  Google Scholar 

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All authors contributed to writing the manuscript after many rigorous discussions.

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Correspondence to James Green.

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Peer review information Nature thanks Morgan Cable and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Green, J., Hoehler, T., Neveu, M. et al. Call for a framework for reporting evidence for life beyond Earth. Nature 598, 575–579 (2021).

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