Imagine an alien civilization that runs on dazzling technologies far more advanced than our own. Picture city-size starships, planet-wide mines or swarms of solar panels that harvest energy directly from a star.
As these aliens go about their otherworldly business, the erosive forces of space wear down their infrastructure, loosening tiny bits of debris that detach and drift into the cosmos. If the civilization collapses, its remains degrade even faster. Clouds of particles might be unleashed into the galaxy, akin to the microplastics in Earth’s oceans, sprinkling countless worlds with the detritus of an extraterrestrial society.
Some of these particles, if they exist, may have even wafted through our solar system, perhaps settling on the surface of the moon, proposes a team of scientists focused on the search for extraterrestrial intelligence, or SETI. In their provocative study, which is in peer review for the International Journal of Astrobiology, they suggest that searching for these microscopic remnants in lunar soil could resolve one of humanity’s great questions: Are we alone in the universe?
“If any other technological civilization has ever existed in our galaxy, it’s not an unreasonable hypothesis to think that particles of them are on the moon in some quantity,” said Lewis Pinault, an affiliate scientist at the SETI Institute in Mountain View, Calif. and a researcher at Birkbeck College, University of London , who led the research.
In the study, the team showed how alien infrastructure might weather and erode into tiny particles and modeled how they would be dispersed through the galaxy. This process is already well observed here on Earth: Satellites, for example, shed microscopic material into space.
Sofia Sheikh, a scientist at the SETI Institute who studies hypothetical alien technologies known as “technosignatures,” described the team’s idea as “a creative and worthwhile avenue.” She said that is especially true in the context of renewed interest in the moon from Artemis, a NASA program that aims to return astronauts to the moon’s surface.
“We know on Earth that time and entropy take their toll on everything that humans make,” wrote Dr. Sheikh, who was not involved in the study, “and there’s a lot more to explore in how the breakdown of technological artifacts can be applied to SETI searches.”
Dr. Pinault and his colleagues modeled the possible trajectories of the particles through the galaxy, showing how they might intersect with our solar system over time. They speculate that some particles might survive impacts with the moon, leaving traces of alien societies that may have already gone extinct.
“If you concede that looking for alien intelligences is a legitimate part of science, which clearly it is, then looking for the technological products — a.k.a artifacts of these intelligences — ought to be just as scientifically valid as looking for their radio emissions,” said Ian Crawford, a professor of planetary science at Birkbeck College, University of London and a co-author on the study.
The idea was inspired in part by the Ukrainian researcher Alexey V. Arkhipov, who in the 1990s pioneered the idea that the moon could be a collection site for alien artifacts. But in that era, the search for alien life was largely relegated to the fringes of academic research, and Dr. Arkhipov’s research didn’t gain much traction at the time.
Dr. Pinault and his colleagues have resurrected the concept and trained their focus on particles on the small scale of microns, comparable in size to a single bacterium. In a nod to Dr. Arkhipov’s work, the scientists named their hypothetical specks “Arkhipov particles” in the study.
Each star could have had multiple civilizations that were generating debris, Dr. Pinault said. He added that aliens “may have become starfarers who are developing similar technologies around each star system that they go to, so the potential of there being quite a bit of interesting debris in the galaxy is pretty high.”
The researchers also considered the possibility that aliens might have dispatched microscopic machines for surveillance purposes or other types of active missions. They called these operational objects “Bracewell particles” after the physicist Ronald Bracewell, who proposed the idea of autonomous alien probes in the 1960s.
Jason Wright, a professor of astronomy and astrophysics at Penn State and a SETI researcher, called the preprint “impressive” and “extremely thorough” and said the approach was worth a shot.
“They’ve revived what is actually a pretty obscure idea and shown that it’s plausible,” said Dr. Wright, who was not involved in the work.
To test their hypothesis, the scientists propose sending a mission to the surface of the moon to sift through about a cubic meter (35 cubic feet) of soil as a starting point — a tall ask. Specialized instruments would then comb through the soil searching for artificial particles, perhaps by screening for metals and other manufactured substances.
Dr. Pinault has already developed models designed to search for larger artificial objects on the moon, such as the unlocated remains of the Soviet lander Luna 9. Similar models could also be trained to detect tiny grains that might have an artificial origin.
Even if artificial grains were identified, there would be a high chance that they were of human origin. Many spacecraft parts have crashed into the moon over the decades, so it would not be surprising to find micron-scale debris from our own defunct missions. To determine whether a particle came from Earth or from some alien world, scientists would need to analyze the chemical fingerprint of the object.
Finding even one alien technosignature in one lunar sample is a long shot, Dr. Crawford said. But if one were found in the moon’s dust — and confirmed to be genuinely alien, rather than rubbish from our own civilization — it would answer the tantalizing mystery of whether life exists beyond Earth.
“The chances of finding anything on the moon are so small,” Dr. Crawford said, that “this would immediately imply that there must probably be lots and lots of them, at least in the history of the galaxy.”
“It would be a very, very important discovery if made,” he added, “but it is a needle-in-a-haystack exercise.”