Ticking the boxes for an alien signal

The signal itself, at 1148.4167 megahertz, checked several boxes that astronomers use to judge whether a radio detection might be artificial rather than natural.

First, it was narrowband. A narrowband signal occupies an extremely thin slice of the radio spectrum, and natural cosmic sources rarely produce them. As the paper states plainly: "Natural astrophysical processes rarely generate extremely narrow spectral features while engineered transmitters often do."

Second, the signal drifted — its frequency shifted gradually over time. That matters because the relative motion of a planet spinning on its axis and orbiting its star can shift a radio signal's frequency through the Doppler effect, the same phenomenon that makes an ambulance siren change pitch as it passes. A drifting signal fits what a transmitter on an orbiting, rotating planet would produce. There is a catch, though: terrestrial interference can sometimes imitate that drift.

Third, the signal appeared in only one spot in the sky. If a candidate signal shows up in multiple locations, it almost certainly comes from Earth-based sources. The K2-155 signal stayed put. That was another box ticked.

Why the evidence points back to Earth

For all that promise, two details pushed the researchers toward a more mundane conclusion. The signal was strong in only one polarization channel of the telescope — polarization refers to the orientation of a radio wave's oscillation, and genuine astrophysical signals typically behave differently across channels. Even more telling, the team found similar signals at three other stars, at nearly the same frequency.

A single alien transmitter on one planet should not produce near-identical signals appearing alongside multiple unrelated stars. That pattern fits earthly radio interference far better.

Outside researchers have reached a similar reading. Prof. Abel Méndez, a planetary astronomer, commented on the results on September 30, 2026: "I looked at the data, and it looks like terrestrial interference."

An open question, but not a hopeful one

Could the signal still be alien? Maybe. But the researchers themselves cannot identify its source, and the weight of the evidence — one polarization channel, similar signals at other stars — points away from an extraterrestrial origin.

The episode highlights both the promise and the difficulty of modern SETI work. Machine-learning tools can now scan hundreds of thousands of candidate detections in data that humans once had to inspect by hand, and they can flag signals that obey the rules expected of an alien broadcast. Yet Earth is a noisy place, radio-speaking, and human-made interference can mimic nearly every hallmark astronomers use to spot an artificial cosmic signal.

For now, the K2-155 signal joins a long list of intriguing detections that remain unexplained but probably terrestrial. The search continues, and the tools keep improving. The next candidate — or the one after that — may one day tick every box without a catch.