The mainstream approach in the search for life, as defined by the Decadal Survey in Astronomy and Astrophysics for the 2020s, is to search for the molecular fingerprints of primitive life forms such as microbes in the atmospheres of exoplanets. These biosignatures can be detected as absorption lines in background light as these planetary atmospheres transit in front of their host stars.
This approach is considered a priority worth investing billions of dollars in future telescopes, because it is widely believed that microbial life is more common than technological life. Indeed, microbial life existed on Earth from its early beginning, about 3.7 to 4.1 billion years ago, whereas advanced technological life emerged only over the last century. As a result, federal funding of the search for technological signatures from extraterrestrial civilizations is down by several orders of magnitude relative to the level allocated to the search for biological signatures.
However, this priority system ignores the stubborn fact that abundance is not equivalent to ease of detectability. Plenty of inconclusive signatures are weaker than a single undisputable detection of a rare signature. The subtle imprint of microbes on the atmospheres of their host planets is challenging to detect from an astronomical distance, whereas a technological signature in the form of industrial pollution or city lights or interstellar objects of technological origin, might be much easier to interpret.
Within a billion years of technological history, a single technological civilization could have launched self-replicating probes that multiplied exponentially and reached remote locations within the Milky-Way galaxy, including the vicinity of Earth. However, the spread of biological entities may be restricted to their home planets, because of the lack of nutrients and the harsh conditions of vacuum and cosmic-ray bombardment in interstellar space.
In order to hedge our bets, it would be prudent to diversify our search portfolio and invest similar funds and effort in the search for bio-signatures and techno-signatures. This is not the dominating view but perhaps it should be. The Galileo Project is following this rationale.
A recent article in Quanta Magazine explained the uncertainties inherent in using molecular fingerprints as biosignatures. Many of these signature molecules, such as oxygen, methane, water and carbon dioxide, but also more exotic molecules such as phosphine or dimethyl sulfide, could potentially be produced by natural chemical processes which do not involve life. The same spectrographs could potentially search for complex molecules that are not produced by nature, such as Chlorofluorocarbons (CFCs) which are produced technologically by refrigerating and aerosol propellants systems on Earth. It might also be appropriate to search for rare isotopes as the byproducts of nuclear reactors on exoplanets.
In a meeting of my research group, some postdocs suggested using Artificial Intelligence (AI) software to help the scientific inference of biosignatures. But I argued that AI is most effective in analyzing data-rich systems, whereas the challenge with molecular biosignatures is that of limited data that can also be reproduced in other ways. Of course, if we acquire a sample of the material on the surface of a planet, we could reach firmer conclusions but such a task is only accessible in the near future within the solar system. Indeed, there are plans to visit Venus, Mars, Enceladus, Titan and Europa, but conditions on habitable exoplanets could be dramatically different than those realized in the solar system.
Given our ignorance, it makes most sense to study the sky agnostically and search with equal vigor for biological and technological signatures. One thing is clear: without searching, we will not find anything.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. His new book, titled “Interstellar”, was published in August 2023.
