Imagine a distant civilization, millions of years more advanced than ours, sending a probe toward our solar system—only for that probe to drift past Earth, unnoticed, mistaken for a tumbling rock. According to astronomer Dr. T. Joseph W. Lazio, this isn't science fiction. It's a distinct possibility we currently lack the tools to rule out.
In a new paper posted to the arXiv preprint server, Lazio, an astronomer at the Jet Propulsion Laboratory, argues that humanity cannot yet definitively prove or disprove the presence of extraterrestrial technosignatures—dead probes, active spacecraft, or abandoned machinery—in our own cosmic backyard. The finding isn't cause for alarm, though. It's a call to keep looking.
To map where we stand, Lazio adapts a four-quadrant framework from a W.M. Keck Institute for Space Studies report, sorting possible alien artifacts by location and condition. Some could be dead wanderers drifting through space on hyperbolic trajectories; others might be active probes zipping between planets. There could be silent wreckage resting on the surface of a moon, or even functioning stations on an asteroid—mining operations or monitoring equipment, quietly doing their work.
The question Lazio tests is stark: Can we falsify the idea that at least one such artifact exists in our solar system today? His answer: not even close.
The challenge isn't necessarily detecting objects—it's proving they aren't natural. Consider 2020 SO, an object detected in 2020 that scientists initially classified as an asteroid. Its orbit was strange, so researchers took a closer look. The near-infrared spectrum matched stainless steel and polyvinyl fluoride exactly—not rock, but a Centaur rocket booster from NASA's 1966 Surveyor 2 mission. It had been orbiting the Sun undetected for over five decades.
"The issue with finding free-floating passive artifacts isn't detecting them outright; it's proving they aren't just one of the millions of other passive rocks floating throughout the solar system," Lazio writes.
The resolution problem compounds this difficulty. Most of the solar system has been imaged at scales that would completely miss anything humanity has ever built. On Saturn's moons, coverage maxes out at roughly one kilometer per pixel. Even our Moon, where we can achieve 0.5-meter resolution, has only been mapped in that detail across a small fraction of its surface.
There are reasons for optimism, though. We already know interstellar probes are possible: humanity has five robots on escape trajectories out of the solar system—Pioneer 10 and 11, Voyager 1 and 2, and New Horizons. They won't be operational by the time they reach other star systems, but they prove the concept works. If we could do it, perhaps others have too.
Active probes, in theory, might be easier to spot—they generate waste heat and must obey thermodynamics, making them stand out against the cold backdrop of space. For now, though, Lazio's paper serves as both a reality check and a roadmap. The search is just beginning, and the universe isn't keeping score of how many questions we haven't answered yet.