One night in April, 1950, nine lights appeared among the stars above Palomar Observatory in California. They had not been seen there before, and haven’t been seen there since. What were they?

The most likely explanation is that they’re a mirage—an illusion created by unknown contamination of the photographic plate taken by Palomar’s Samuel Oschin Telescope. But If contamination or mistakes can be ruled out, then there’s another startling possibility: that the transient light sources were glints of sunlight reflecting off metallic objects in near-geosynchronous orbit.

This was more than seven years before the first artificial satellite, Sputnik 1, was launched by the Soviet Union. If those glimmers of light do belong to a swarm of passing satellites, then they are not of human construction. Instead, they could be the first evidence for extraterrestrial, technological intelligence.

Those nine lights—referred to as transients because they came and went—were discovered by astronomers working on the VASCO (Vanishing and Appearing Sources during a Century of Observations) project, which is led by Beatriz Villarroel of Stockholm University. The idea is to scrutinize old photographic plates of the night sky taken at observatories such as Palomar, and compare them to modern images to see if anything has changed—have any faint stars, which might otherwise have gone unnoticed, appeared or disappeared in that time?

If they have, then they could be evidence for all manner of astrophysical phenomena, including the afterglows of gamma-ray bursts and kilonovae, or eruptions from stellar mergers. However, the probability of nine cosmic explosions occurring all at the same time so close together in the sky is very low.

It’s not been lost on the VASCO team that their project also has a SETI dimension. A disappearing star could be evidence for highly advanced extraterrestrial technology, such as some form of Dyson sphere, or a mysterious light could be a reflection from an alien probe present in our Solar System.

The idea that technological aliens might have sent probes out into the Universe is not an outlandish one, but in fact dates back to 1960, the same year that modern SETI began. As radio SETI was finding its feet in the pages of Nature and at the Green Bank Observatory thanks to Cocconi, Morrison, and Drake, Australian scientist Ronald Bracewell of Stanford University went against the grain to propose an alternative: that aliens might eschew radio contact and, instead, venture forth to explore the Universe themselves using probes.

They became known as Bracewell probes, and a variety of types and functions have been hypothesized over the years, from von Neumann’s self-replicating probes to science fiction author Fred Saberhagen’s destructive Berserker probes. The concept of sending robotic spacecraft to explore the Universe now seems a natural one to us, given the number of space probes that we have sent to the other planets in the Solar System. NASA's twin Voyager spacecraft were launched in 1977 with a greeting from Earth to an intelligent alien civilization and both are currently soaring past the outer boundary of the heliosphere in interstellar space, continuing to transmit useful data to Earth with no ultimate destination.

There’s even discussion of sending probes on interplanetary missions beyond our Solar System, from design concepts such as Daedalus, to the nano-sized StarChips of Breakthrough Starshot. So, why wouldn’t aliens look to do the same?

Of course, in lieu of a warp drive, interstellar travel must take the slow road. Even so, Breakthrough Starshot calculated that their probes could reach Alpha Centauri in less than 25 years, cruising at up to a fifth of the speed of light. However, to venture further afield and explore the entire Milky Way Galaxy would take millions of years.

That time dimension works in our favor, however. Life in the Universe could be billions of years older than we are, and there’s been plenty of time for other civilizations to explore our Galaxy. Perhaps they sent a probe, or probes, to study Earth long ago. Those probes could still be in orbit today, perhaps intact, perhaps still active, or perhaps having long ago gone offline and now nothing more than a cloud of disintegrating parts.

It’s this possibility that has sparked Beatriz Villarroel’s interest, and she, along with others, are now exploring ways to search for these probes, or their remnants, which are referred to as non-terrestrial artifacts, or NTAs.

“The simplest way, in my opinion, to search for NTAs is in photographic plates taken before the Space Age,” she tells Supercluster. They cannot then be confused for human-built satellites, of which there are over 7,900 currently in orbit, plus the millions more pieces of space debris if they were captured in photographs taken before humans launched a single satellite.

The search for extraterrestrial technological artifacts, or SETA as it’s being referred to, is SETI’s latest frontier, with researchers growing increasingly confident about discussing the possibility that our first evidence of technologically intelligent extraterrestrial life may come in the form of technology that has traveled to our Solar System.

“The times are changing,” says Villarroel. “The search for NTAs is a valid area of research.”

This view is echoed by Lewis Pinault, who is a researcher with University College London’s Centre for Planetary Sciences, and a partner at Airbus Ventures. Initially trained as a meteoritics scientist with sponsorship from NASA, at Airbus Ventures Pinault works with dozens of aerospace start-ups, many of which use advanced materials, some made from high-performance molecules developed with the assistance of artificial intelligence. For example, IBM recently constructed, atom by atom, a dense, novel compound called triangulene, which is too unstable to form naturally, but which has quantum properties that might be useful for electronics and other sophisticated machinery. Thousands of other such molecules have been developed.

This explosive molecular development, and the potential application of those molecules in aerospace technologies, has inspired Pinault to think about how such molecules might offer clues to the existence of extraterrestrial technology in our Solar System.

“The potential for finding an NTA may be greater than detecting an alien radio signal,” says Pinault.

He isn’t necessarily looking for complete extraterrestrial spacecraft, but is instead focused on searching for technology at sub-micron scales, in the guise of those novel, high-performance molecules. He reasons that a species no more technologically advanced than us might have begun mining asteroids in their home planetary system, producing voluminous clouds of debris that could contain traces of advanced technology. “Think of exotic metals in drill bits that spall off tiny traces as they bite space rock,” suggests Pinault.

Or, a species slightly more technologically advanced might have seeded the galaxy with ‘programmable matter’—nanomachines capable of turning raw asteroidal material into something more useful, such as larger probes perhaps designed to study planetary systems or to engage with any life that the probes discover.

Whether it be dormant nanomachines or metallic flakes from drill-bits, Pinault’s hypothesis, building upon research begun in the Soviet Union in the 1980s, is that over the eons some of this tiny artificial detritus could have arrived in interstellar dust and fallen upon the Moon or other airless bodies of our Solar System, where it may still lurk, hidden in the lunar regolith. It’s even possible that remnants of extraterrestrial probes, long since powered down and churned back into the regolith by space weathering and constant micrometeorite impacts, could be poking up above the surface of the Moon or other similar bodies.

Driven by these possibilities, Pinault is developing ways that such remnants of extraterrestrial technology could be distinguished from naturally occurring molecules and features on the Moon and asteroids.

“The chances might be just one in a trillion, but machine learning and nano-scaled detection systems make this a surprisingly tractable task,” says Pinault. “We shouldn’t rule out well-preserved environments on Earth, either – studies of ancient micrometeorites strewn across the Antarctic deserts might be surprisingly productive, too."

Indeed, up to 100 tons of micrometeorites fall to Earth each day. So who knows––a typical school-level experiment to collect and identify space dust with plastic tubs and magnets could potentially contain evidence of extraterrestrial technology from long ago and far, far away.

Supposing that anomalous molecules that have seemingly been manufactured are discovered, Pinault says that they “could probably be radioisotope dated. If we find something that suggests ‘someone made me’ and it’s evidently older than we are, then I think we’ve arguably found our first [evidence for] intelligent extraterrestrial life.”

Or perhaps not. There’s another possibility, which is that such materials could have come from civilization on Earth—but not our civilization.

Its originators, astronomer Adam Frank of the University of Rochester and Gavin Schmidt, a renowned climate scientist at NASA’s Goddard Space Flight Center, call it the Silurian Hypothesis. They asked themselves the question, could another intelligent species, another civilization unrelated to Homo sapiens, have existed on Earth hundreds of millions, or billions, of years ago?

Implications for the fossil record and the tree of life notwithstanding, Frank and Schmidt concluded that it is feasible.

This isn’t the same as saying that they have evidence for such a civilization. No such evidence has been found, and Frank and Schmidt aren’t claiming to have found any. Rather, they have shown that most, if not all the evidence for an ancient civilization could have long since been eroded or weathered away, meaning we’d have no way to say whether they could have existed or not. Think about all the detritus of our modern industrial society, and how we have marked the environment, from our mountains of trash to atmospheric pollution. Yet human cities only cover one percent of Earth’s surface, and after erosion and tectonic forces have wiped the surface clean, and pollution has been washed out of the atmosphere, there wouldn’t be much evidence left on geological time spans.

But the vacuum of space offers a much better environment for preserving evidence for ancient civilizations, be they from Earth or beyond. In a 2018 paper, Hector Socas-Navarro of the Instituto de Astrofísica de Canarias in Tenerife, described the concept of a ‘Clarke Exobelt’ (named after Arthur C. Clarke), which is a ring of satellites in geostationary or geosynchronous orbit. Socas-Navarro was investigating whether we could detect a Clarke Exobelt around another planet, as a kind of technosignature (Socas-Navarro concluded that yes, we could), but more pertinently to this discussion, he calculated that objects could survive in orbit in an Exobelt for a minimum of tens of thousands to tens of millions of years, and potentially at most for billions of years.

Consequently, any NTAs in orbit around Earth may have been there a very long time.

Earth orbit isn’t the only location in which an NTA might lurk. James Benford has proposed that co-orbiting asteroids, which orbit the Sun in time with the Earth, would be attractive locations for probes to keep an eye on our planet. Meanwhile, John Gertz has provided an overview of where NTAs may remain hidden in the Solar System, including the Lagrange points and the nearside of the Moon, from where an NTA would be in a great position from which to observe our planet. With programmable nanoprobes in mind, Pinault is exploring this latter possibility by developing search algorithms to distinguish artificial constructs from natural landforms on the lunar surface seen in high-resolution imagery from NASA’s Lunar Reconnaissance Orbiter.

But what of those nine lights? Villarroel’s team searched for them in the same location on the sky as the photographic plate, using the mighty Gran Telescopio Canarias, which is currently the largest ground-based optical telescope in the world, but they found no convincing evidence that the objects behind those lights were still present in the field of view. If the lights did belong to NTAs, could their orbits be calculated to predict where they are now?

“It would be extremely challenging, even though there are methods to calculate their orbit and predict when we will be able to observe the glints again,” says Hichem Guergouri of the Research Unit in Scientific Mediation in Algeria, and a colleague of Villarroel’s on VASCO. “Given the single epoch of observations and the 70 years in between, we cannot expect any accuracy more than guesswork."

However, the story isn’t over yet, says Villarroel. “If the nine transients are real observations caused by space debris, then we expect to see other signatures of space debris in the old photographic plates, for example, several simultaneous transients aligned along a straight line.”

There remain some cautionary points. One possible source of contamination of the photographic plate is radioactive fallout from nuclear weapons tests in New Mexico during the period of the late 1940s and early 1950s. However, there were no official tests between 1949–51, which places a question mark on this hypothesis.

Then there’s the ‘Face on Mars’ syndrome – the danger of seeing what we want to see, especially when searching for artificial objects on planetary bodies such as the Moon. Pinault recognizes this risk, but emphasizes that the peer-review process should be able to sort reality from our hopeful biases.

Should a bonafide NTA be discovered, it would change everything. The knowledge that there were aliens in our backyard, no matter how long ago, would be disquieting, but the knowledge that we are not alone in the Universe might also be uplifting. There would undoubtedly be a race to retrieve the probe and discover its secrets, which is why the international community should start considering putting protocols in place to avoid conflict over any extraterrestrial technology, just as there are protocols in place for what should happen if an extraterrestrial radio signal was discovered.

For Villarroel, the benefits would be more prosaic. “Nearly all of our approximately 40 members in VASCO work pro bono,” she says. “I hope that VASCO, with its extensive research program, will be able to secure funding with time.”

Maybe those nine lights will remain an unsolved mystery, destined to join the Wow! signal in the annals of SETI legend. But should they turn out to be the real deal, It’s difficult to contemplate the full scope of resulting societal, political, and economic implications.

Whether our civilization is ready is another story.