Scientists searching for evidence of extraterrestrial technology have identified an unexplained radio signal coming from the direction of a star system approximately 238 light-years from Earth. Designated NBS 260108, the narrowband signal was discovered when a Chinese-led research team tested new data-processing software on observations collected in 2021 by China’s Five-hundred-meter Aperture Spherical Telescope, known as FAST.

The observations were aimed at K2-155, a star system that contains three known planets larger than Earth but smaller than Neptune. One of those planets, K2-155 d, is a so-called super-Earth located near the star’s potential habitable zone, making the system an interesting target in the search for extraterrestrial intelligence.

The signal was recorded at approximately 1,148.4167 megahertz and appeared as a continuous radio tone concentrated within an extremely narrow frequency range. Such signals attract attention in the search for technological activity because natural astrophysical processes rarely produce similarly narrow frequency patterns, while engineered transmitters can generate them.

Researchers also observed that the signal’s frequency gradually decreased over time, drifting at approximately 0.038 hertz per second. A changing frequency can result from the relative motion between a transmitter and receiver, producing a Doppler shift that astronomers examine when evaluating possible signals from distant technological sources.

Another feature that initially made the signal stand out was its apparent location within FAST’s 19-beam receiver system. The signal appeared in the beam pointed directly toward K2-155, but researchers found no corresponding detection in the other 18 beams. This distinction matters because radio interference from sources on or near Earth can often appear across multiple beams, whereas a signal originating from a distant object may be confined to the beam observing that location.

However, the researchers identified an unusual polarization pattern that complicates the possibility of an extraterrestrial origin. FAST records radio signals through two polarization channels, labeled XX and YY. NBS 260108 appeared strongly in the YY channel, with a signal-to-noise ratio of approximately 23, but was absent from the XX channel. The researchers noted that such an extreme imbalance can be a warning sign of interference originating from the telescope's equipment or human technology.

Further examination revealed similar YY-dominated radio features in observations of three other star systems recorded during the same observing session: K2-18, GJ-9066 and Ross 128. Although those signals differed in strength, frequency and drift characteristics, their appearance in the same polarization channel and nearby frequency range raised additional concerns about a common interference source.

The team also examined possible sources of radio-frequency interference, including satellite communications, navigation systems, aviation equipment and military transmissions. None provided a definitive explanation for the signal, and the researchers acknowledged that its precise origin remains unresolved.

In their study, published in The Astronomical Journal and available through the scientific preprint repository arXiv, the researchers cautioned against treating the finding as evidence of extraterrestrial intelligence. They concluded that an artificial or instrumental source on Earth is more plausible than a signal originating from K2-155, classifying NBS 260108 as a low-priority candidate for additional investigation rather than a confirmed detection.

The researchers said future observations and more detailed comparisons across FAST's receiver beams and polarization channels could help determine where the signal originated. For now, the discovery demonstrates both the potential of increasingly sophisticated search techniques and the difficulty of distinguishing possible extraterrestrial transmissions from interference generated by human technology.