Mysterious Radio Signal from K2-155: Alien Message or Instrumental Interference?

The K2-155 radio signal now circulating as a possible alien message is a single narrow tone at 1,148.4167 megahertz. China’s FAST telescope recorded it in 2021 while pointed at the K2-155 system, 238 light-years away. The scientists who found it don’t think it came from there. In their peer-reviewed paper in The Astronomical Journal , published Aug. 27, the team calls the signal “a low-priority technosignature candidate” rather than a firm detection. They add that an “anthropogenic or instrumental origin” is “more plausible than an extraterrestrial one.” The full text is free on arXiv . That matters because the claim has been recirculating for weeks under “alien life?” headlines. Here is what the paper shows, and why its discoverers moved the signal down their own list. The signal was not picked up live. Zi-Qi Li of Beijing Normal University and 16 co-authors re-ran old data through new software. Most are at Chinese institutions, with one co-author affiliation at the University of Glasgow. The data came from 2021 observations of 33 planetary systems. They were made with FAST, the Five-hundred-meter Aperture Spherical radio Telescope in Guizhou province. Their pipeline uses wavelet analysis and machine learning to hunt for narrowband signals, the kind engineered transmitters make. The reasoning, as ScienceAlert quoted it, is that “natural astrophysical processes rarely generate extremely narrow spectral features while engineered transmitters often do.” Under the team’s chosen settings, the software produced 139,127 single-beam detections. A veto that throws out anything seen across several beams cut that to 803 candidates. Further checks and a visual inspection left two notable events. One pointed at Kepler-438, an old candidate this same research group reported in 2022. The other was new and got the label NBS 260108. It came from a scan of K2-155 on Sept. 10, 2021. Three features made the K2-155 tone worth a second look. It was extremely narrow. It slid steadily down in frequency, by about 0.038 hertz per second. A transmitter on a moving, spinning planet could produce that kind of drift. And it appeared in only one place. That last test is FAST’s strength for this work. Its receiver has 19 beams, so while beam 1 stares at the target, the other 18 watch nearby sky. A radio source on the ground or a passing aircraft usually shows up in several beams at once. NBS 260108 was in beam 1 only. The target itself also explains the attention. K2-155 is a small, cool star hosting three known super-Earths. The paper puts the outermost, K2-155 d, near the habitable zone, with a 40.7-day orbit and a radius about 1.6 times Earth’s. NASA’s exoplanet catalog lists a larger radius, 1.9 times Earth’s, and estimates the mass at 4.27 Earths. “Near the habitable zone” says nothing about whether anything lives there. Two findings in the paper pull hard the other way. Radio receivers record two polarization channels, labelled XX and YY. NBS 260108 was strong in YY, with a signal-to-noise ratio of about 23, and absent from XX. The authors flag that lopsided pattern as a possible warning sign of instrumental or terrestrial contamination. The second clue is the company it kept. K2-155 was observed in a sequence that night with K2-18, GJ 9066 and Ross 128. All three showed YY-dominated features near the same frequency. The authors say such a repeat “strongly favors a common interference origin over a signal located at K2-155.” Across the whole survey, similar comb-like structures turned up in 17 of the 33 star observations. The team also checked the usual suspects in this band: aircraft radio beacons, satellite communications, GNSS positioning satellites and military radar. None fit cleanly. Aviation and radar signals tend to be pulsed or hopping, and satellite signals don’t explain the polarization. So the paper leaves the source “unclassified.” Abel Méndez was blunter. He directs the Planetary Habitability Laboratory at the University of Puerto Rico at Arecibo. “I looked at the data, and it looks like terrestrial interference,” he wrote on Bluesky on Sept. 30, in a post EarthSky embedded in its coverage. Reading the new paper beside the group’s 2022 work shows a pattern. In June 2022, a FAST signal toward Kepler-438 drew “alien” headlines. It was first reported by the South China Morning Post, Futurism noted . The 2022 paper was led by Zhen-Zhao Tao, with Tong-Jie Zhang as a co-author. Published in The Astronomical Journal , it concluded that the signal’s “polarization characteristics are almost able to eliminate the possibility of an extraterrestrial origin.” The new pipeline recovered that Kepler-438 signal, at 1,140.604 MHz, and measured it again. It came out with a signal-to-noise ratio of 23.2 in YY and 4.4 in XX. That is the same one-channel signature now seen in NBS 260108, about 8 MHz away in frequency. Our reading: two “best candidates” from one telescope share that fingerprint. That points to the local radio environment or the instrument, not two unrelated star systems. It also resembles Breakthrough Listen’s BLC1, a 2019 narrowband signal seen toward Proxima Centauri with Australia’s Parkes telescope. Analysis published in 2021 traced it to human-made interference. “BLC1 is, sadly, not a signal from intelligent life beyond Earth,” Danny Price of Curtin University wrote in The Conversation . The new paper notes BLC1 drifted at plus 0.038 Hz per second. That is the same rate as NBS 260108, in the opposite direction. The paper says a definite answer “requires additional observations capable of establishing a direct emitter association,” meaning catching the interfering transmitter in the act. For now NBS 260108 is kept “only as an internal follow-up item for future observations.” The team also plans to work with FAST’s raw voltage data from all 19 beams. Cross-correlating the beams could show where a signal sits in the sky and whether it moves. That is the test most likely to separate a distant star from a nearby source. Other candidates are on the same footing. Rubin Observatory’s possible dwarf planet beyond Pluto also waits on follow-up. Meanwhile the most distant spacecraft still talking to us is Voyager 1, now nearly a light-day from Earth . If the tone reappears from K2-155 alone, in both polarization channels and on a different night, it becomes a real story. The authors have not announced a date to point FAST back at K2-155.