The task of astrobiologists who seek evidence for extrasolar microbes is as simple as imitation. Primitive forms of extraterrestrial life should exist under physical conditions that were nurtured in natural environments similar to those on Earth. Hence, astrobiologists plan to search for the molecular products of microbes-as-we-know-them on rocky exoplanets with atmospheres in the habitable zone of their host stars.
The latest Decadal Survey in Astrophysics of the National Academies, Astro2020, recommended a 6-meter space telescope operating in the infrared, optical and ultraviolet bands, capable of high-contrast imaging and spectroscopy. Scheduled for launch by NASA in the 2040s, this Habitable Worlds Observatory will be designed to search for spectroscopic biosignatures of microbial life in roughly 25 habitable zone exoplanets at a likely cost of over 10-billion dollars.
Where to look becomes more subtle when dealing with intelligent forms of life because they can relocate and spread away from their birth exoplanet. Almost all forms of life, on Earth and beyond, died by now. Those who survive the longest on our cosmic street might have done so by escaping their home planet on artificially made space platforms to avoid existential risks from local catastrophes.
As much as nature is often viewed as a blessing, it could also be a curse. For example, there is strong evidence that Earth’s surface became nearly entirely frozen with no liquid water exposed to the atmosphere during the Cryogenian period, more than 650 million years ago. Over the entire history of Earth, more than 99.9% of the over 5-billion speciesthat ever lived went extinct. Some of these species were wiped out by well-documented cataclysmic events, such as the Cretaceous–Paleogene boundary event which was likely triggered by the impact of the Chicxulub meteor which eliminated 75% of plant and animal species on Earth including the non-avian dinosaurs. However, the majority of extinctions could have been the result of natural evolution with the livelihoods of some species destroyed by changes and unfavorable competition. The situation resembles political views which lose their livelihood after a devastating Presidential election.
The typical lifespan of a terrestrial species is 1-10 million years. This is an interesting factoid, since humans evolved from great apes through the lineage of hominini – which arose 5-7 million years ago. Keeping these facts in mind, the human species is getting close to the end of its expected natural lifespan. Will technology save us?
As much as technology could delay the existential threats from a snowball or a globally-warmed Earth, the evolution of the host star itself sets a time limit on the ability of a natural nuclear furnace to support life on a planet. Most stars formed billions of years before the Sun, and the Sun will boil off all oceans on Earth within a billion years. Recognizing this and other existential risks, a sufficiently advanced technological civilization would prefer to migrate away its birth habitat. This rationale inspires Elon Musk’s vision to occupy Mars and make humanity a multi-planet species.
However, once a technological civilization develops the ability to migrate away from its birth planet, it might also be capable of pursuing its goals far away from its host star on an artificial space platform, potentially powered by its own nuclear energy source. This could lengthen considerably the most important parameter in the Drake equation, namely the lifespan of a technological civilization.
Traditionally, the search for extraterrestrial intelligence (SETI) targeted the vicinity of stars. However, once technology allows escape from natural habitats, all bets are off. Space platforms carrying biological entities or hardware with artificial intelligence may be found anywhere. Technological probes that can self-replicate could fill up interstellar space in less than a few billion years using conventional chemical propulsion.
Given this realization, it makes most sense to start the search for technological debris around the nearest lamppost, our Sun. This is the rationale behind the Galileo Project, which under my leadership searches for technological artifacts within the orbit of the Earth around the Sun. Our research team operates a new observatory at Harvard University and constructs two additional observatories in Pennsylvania and Nevada. My students and postdocs also plan to analyze forthcoming data from the Rubin Observatory in Chile and the Webb telescope in the coming years. Finally, we hope to conduct expeditions in search for materials from crash sites of interstellar meteors.
What other civilizations accomplished may exceed our imagination. Therefore, it is best not to anticipate what we might find but instead search for unfamiliar objects, not produced by nature or human-made technologies.
We must hedge our bets in the search for life. This means investing billions of dollars not only in seeking biological signatures of microbes in atmospheres of exoplanets but also in the search for technological artifacts that arrive near the Sun from interstellar space. With all these efforts, the keys of life might first be discovered around the nearest lamp post on our cosmic street.
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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.