• Narrowband signal picked up from direction of the Kepler-438 star meets initial criteria for extraterrestrial intelligence, preprint says
• Orbiting Kepler-438 in its habitable zone is one of the most Earthlike planets ever found outside the solar system

Alien-hunting astronomers in China and the US say they discovered a mysterious radio signal from the direction of an Earthlike planet.

The narrowband signal – picked up by the world’s largest radio telescope – was “meaningful”, though further confirmation was needed, the scientists said.

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The candidate signal came from the direction of Kepler-438, a red dwarf star in the constellation Lyra – some 473 light years from the Earth, according to the team’s manuscript submitted to Research Square, a preprint service where researchers share unpublished work for community feedback.

Orbiting Kepler-438 in its habitable zone is Kepler-438b, one of the most Earthlike planets ever found outside the solar system.

The detection was made between November 2020 and September 2021, when the Five-hundred-metre Aperture Spherical radio Telescope (FAST) in southwestern China carried out its first targeted "search for extraterrestrial intelligence" - or SETI - project.

Specifically, FAST was looking for "narrowband" radio signals - of the type widely used in human electromagnetic communications. They indicate the presence of technological sources in outer space, because such signals cannot be produced by any natural astrophysical process, and can only arise from either intentional transmission or leakage, according to the draft paper.

FAST looked at 33 planetary systems in all, of which 29 are known to host planets in habitable zones and five are in the Earth Transit Zone - a special region in space where an extraterrestrial observer would be able to see the Earth pass in front of the sun.

The candidate signal was detected at 1.14 gigahertz by the FAST telescope's 19-beam receiver, which allows simultaneous observation of 19 regions in the sky. All 19 beams on the receiver were recording data simultaneously at the time, but only Beam 1 pointed to the target.

"This is the only (radio) event that was only present in Beam 1 and not in any other beam, which makes it different from any other event we detected," the study said.

The signal persisted during the 20-minute observation period, with slight drifts in frequency usually caused by orbital or rotational motions of celestial bodies, astronomers from Beijing Normal University, the National Astronomical Observatories of China, the University of California, Berkeley and other institutions studying the data found.

Based on these two phenomena, the researchers excluded the possibility of all ground-based radio frequency interference sources outside the telescope, including aeroplanes.

They also ruled out the possibility of artificial objects, such as satellites or space probes. This was because there were no satellite or deep-space probes within the main detection area of Beam 1 at the time, the article said.

SETI is one the five core goals listed in the original project plan of FAST. Before FAST was constructed, scientists would use radio telescopes around the world - including the 305-metre-wide Arecibo Telescope in Puerto Rico - to identify candidate signals, but none has been confirmed to have come from an alien civilisation.

FAST's high degree of sensitivity, wide sky coverage, and highly efficient 19-beam receiver allow it to conduct some of the most sensitive SETI observations, lead author Zhang Tongjie from Beijing Normal University said.

Before FAST officially joined the search for aliens in September 2020, it had already completed its first SETI survey and identified two groups of candidate signals, according to another US-Chinese study published in The Astrophysical Journal in 2020 that was also led by Zhang.

However, the biggest challenge for FAST and other radio telescopes has been to identify and eliminate various man-made and astrophysical noises from the signals received, including for the newly detected one from the Kepler-438 region.

Although it was consistent in some aspects with what a true extraterrestrial signal should look like, "there is still a piece of evidence leading us to suspect that the Kepler-438 event is an instrumental radio frequency interference signal," the researchers wrote.

To investigate further, FAST has launched additional observations to look in the direction of Kepler-438, Zhang, the SETI project lead at FAST, told the state-run Science and Technology Daily on Tuesday.

"It might take a long time to prove it one way or the other, but, even if the signal turns out to be some noise, it will still provide meaningful lessons for our SETI research in the future," he said.

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