Mysterious radio signal potentially from a distant 'Super-Earth' puzzles scientists—is it alien?

Astronomers have been looking for signs of alien life for decades, and the Search for Extraterrestrial Intelligence (SETI) recently experienced a brief surge of excitement. A team of researchers in China detected a mysterious radio signal originating from the direction of a distant exoplanet that is orbiting a red dwarf star named K2-155, about 238 light-years away, hinting at a potential technosignature. This planet, named K2-155 d, is called a ‘Super-Earth’ because it is 1.6 times the radius of our planet and could be potentially habitable, meaning liquid water could exist on its surface under favorable conditions.

The signal was reportedly found among 139,127 candidate signals during a survey of 33 planetary systems. The signal itself was initially captured in 2021 by the Five-hundred-meter Aperture Spherical Telescope (FAST), but it was recently brought into consideration when the research team was re-evaluating the data using new machine learning techniques.

Why the finding excited scientists

When scientists want to confirm if a radio signal is actually being produced by an alien civilization, they look for specific markers to prove it is indeed unnatural and not simply radio interference produced by equipment on Earth. As the researchers narrowed down the list of aforementioned signal candidates, they found two targets most interesting. One signal coming from the direction of the star Kepler-438 turned out to be radio interference, leaving K2-155 d as the primary target of interest.

Its suspected signal had several intriguing characteristics. Radio signals are produced by natural astronomical sources across a broad range of frequencies, but the one radiating from the direction of the Super-Earth exoplanet was found to be very narrow. This hints that it may have been produced by something unnatural, like an artificial transmitter. "In radio SETI, the most widely pursued targets are narrowband drifting signals, since natural astrophysical processes rarely generate extremely narrow spectral features while engineered transmitters often do," the authors wrote in the paper published in The Astronomical Journal. Another characteristic they noted is that the signal was drifting, which is consistent with having a source (an engineered transmitter) on a rotating planet which is orbiting a star. This happens due to a phenomenon called the Doppler effect. Moreover, the signal appeared to emerge from just one location instead of originating from multiple sources in the sky.

Is the signal coming from aliens?

While the above-mentioned facts make the planet an interesting candidate, there are a couple of factors that suggest it was a false alarm. The scientists noted that the signal was strong only in one polarization channel of the telescope, and that they found signals close to the same frequency as the one near K2-155 d from three other stars. This doesn’t mean there are alien civilizations emitting radio signals from three other planets; it most likely means that this was just Earth-based radio interference. In the paper, the authors wrote that their machine learning pipeline can produce up to 2.7 million detections under conservative settings, and some terrestrial interference can perfectly mimic narrow, drifting signals coming from an exoplanet.

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