A New Direct Method to Search for Life Near Other Stars
The top priority of the U.S. astronomy community for the coming two decades was defined in the 2020 Decadal Survey as the Habitable Worlds Observatory. This space mission will survey the atmospheres of potentially habitable exoplanets for the molecular fingerprints of microbial life, at a cost of over 10 billion dollars. Given the astronomical price tag, one may wonder whether there is a better way to acquire this knowledge at a lower cost.
Instead of observing exoplanetary systems at great distances, it occurred to me that we can study closely the composition of interstellar objects that were ejected from them billions of years ago and are now reaching the inner solar system. Most stars formed billions of years before the Sun, and so the objects they ejected to interstellar space had plenty of time to reach our cosmic backyard.
On September 24, 2023, NASA’s OSIRIS-REx mission returned to Earth about 120 grams of pristine carbonaceous regolith from the Solar system asteroid Bennu. Two recent papers (posted here and here) detailed the results from an early analysis of the physical, chemical, and mineralogical properties of the returned material. Among the most exciting detections were amino acids — 14 of the 20 that life on Earth uses to make proteins — and all five nucleobases that life on Earth uses to store and transmit genetic instructions in more complex terrestrial biomolecules, such as DNA and RNA, including how to arrange amino acids into proteins. Although these building blocks have been found before in other Solar-system rocks that landed on Earth, identifying them in a pristine sample collected in space supports the idea that objects far from the Sun could have seeded life-as-we-know-it.
On July 1, 2025, a new interstellar object 3I/ATLAS was discovered. Its discovery inspired me to think about a new direct method to find evidence for life in exoplanetary systems. Instead of the expensive and technologically-challenging Habitable World Observatory, astronomers could design and launch an analog of the OSIRIS-REx mission that will land on interstellar objects, like 3I/ATLAS, and bring a sample of their surface material back to Earth. This constitutes a new way to check if planetary systems around other stars developed the same building blocks of life-as-we-know-it.
For a sample return mission to succeed, the interstellar object must be discovered by a survey telescope like the new Rubin Observatory early enough, many months in advance, to allow sufficient time to rendezvous with it with a maneuvering chemical rocket parked in the inner solar system. Interstellar objects typically move at a high speed, in excess of the escape speed from the Sun — which is 42.2 kilometers per second at the Earth-Sun separation. However, if we are lucky to be visited by a near-Earth interstellar object which approaches the Earth in the direction of Earth’s motion around the Sun at 29.8 kilometers per second, its velocity relative to Earth can be as small as 12.4 kilometers per second, similar to the escape velocity from Earth, 11.2 kilometers per second, that space missions often reach. The required launch speed in this case would need to be 16.7 kilometers per second. The retrieval and return of interstellar material should be performed quickly since interstellar objects spend a short amount of time in the vicinity of Earth.
The cost of the OSIRIS-REx mission was estimated at $1.16 billion, a factor of ten lower than the minimum cost of the Habitable World Observatory. In principle, a landing mission could also target an object manufactured by an extraterrestrial technological civilization, which according to another essay I wrote today, can be distinguished remotely from interstellar rocks. This would offer the benefit of reverse engineering technologies that we had never imagined before because our modern science is only a century old.
Spectroscopy of reflected sunlight or infrared emission from the surface of interstellar objects could also provide clues about its composition. A week after the discovery of 3I/ATLAS, two preprints (posted here and here) reported that its observed spectrum shows evidence for a significant reddening of the reflected sunlight. Such reddening could be indicative of dust, or it might otherwise be related to the surface properties of 3I/ATLAS. For example, Kuiper belt objects in the outer Solar system are reddened when organics on their icy surface are exposed to ultraviolet light or cosmic rays for billions of years. This is caused by Tholins, a wide variety of organic compounds formed by ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon dioxide (CO_2), methane (CH_4) or ethane (C_2H_6), often in combination with nitrogen (N_2) or water (H_2O).
In principle, life could have been delivered to Earth by rocks from another star through interstellar panspermia, as I suggested in a paper with my former postdocs Idan Ginsburg and Manasvi Lingam.
The size of 3I/ATLAS is unknown but based on a new paper that I wrote last week, its diameter is expected to be in the range of 2–20 kilometers, bigger than the estimated diameter of 2I/Borisov — a few hundred meters, or `I/`Oumuamua — about a hundred meters. Based on another paper that I wrote with Manasvi Lingam, the core of large objects could have been warmed by radioactive decays to maintain microbial life and potentially survive an impact on Earth — similarly to the Martian meteorite ALH84001.
These considerations inspired me to do a simple calculation. I figured out that during the 4.5-billion-year lifespan of Earth, there may have been several tens of collisions with interstellar objects like 1I/`Oumuamua or 2I/Borisov but only a 10% chance for a collision with the rarer population of 3I/ATLAS. Impacts by smaller objects, on the sub-meter scale of ALH84001 or the interstellar meteors IM1 and IM2, were obviously much more numerous.
After realizing that, I had a zoom call with my Harvard College student, Shokhruz Kakharov, which led to a paper with new insights on the prospects for interstellar transfer of life. Science can be exciting when you open your mind to the new possibilities allowed by observations of nature. For a detailed account of the exciting new results, stay tuned for our upcoming paper!
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. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
