Scientists in China and the US believe they have discovered a mysterious radio signal coming from the direction of a planet similar to Earth. Scientists said the narrowband signal - caught by the world's largest radio telescope - was 'meaningful', though additional confirmation was needed.

Based on a manuscript submitted to Research Square, a preprint service where researchers share unpublished work for community feedback, the candidate signal came from Kepler-438, a red dwarf star in the constellation Lyra - 473 light-years from Earth.  

Kepler-438b, one of Earth's closest neighbors outside our solar system, orbits Kepler-438 in its habitable zone. The first SETI project to target the Five-hundred-metre Aperture Spherical Radio Telescope (FAST) in southwestern China made the detection between November 2020 and September 2021.  

FAST specifically searched for 'narrowband' radio signals - that type of signal widespread in human electromagnetic communication. As the draft paper notes, these signals are not produced naturally by astrophysical processes and can only arise either from intentional transmission or leakage from outer space.

A total of 33 planetary systems were examined, 29 of which are believed to host habitable planets, and five are thought to be in the Earth Transit Zone - a region in space where an extraterrestrial observer could view Earth passing in front of the sun.  

FAST's 19-beam receiver detected the signal at 1.14 gigahertz, allowing simultaneous observations of 19 regions in the sky. During that time, all 19 beams on the receiver were recording data simultaneously, but Beam 1 was pointing at the target. The study noted that this was the only radio event detected that was only present in Beam 1 and not in any other beams.

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

Due to these two phenomena, researchers ruled out any ground-based radio frequency interference source outside of the telescope, including aircraft. Artificial objects, such as satellites and space probes, were also ruled out. A satellite or deep-space probe was not present within the main detection area of Beam 1 at the time, according to the article.

In FAST's original project plan, SETA was one of five core goals. The Arecibo Telescope in Puerto Rico, 305 m wide, was used to identify candidate signals before FAST was built, but no alien signal has been confirmed.

Lead author Zhang Tongjie from Beijing Normal University said that FAST can carry out some of the most sensitive SETI observations due to its high sensitivity, wide sky coverage, and highly efficient 19-beam receiver.

As part of another US-Chinese study published in the Astrophysical Journal in 2020 that was also led by Zhang, 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.  

The greatest challenge for FAST and other radio telescopes has been identifying and removing various man-made and astrophysical noises from signals, including the newly detected signal from Kepler-438.  

Despite its similarities to an extraterrestrial signal in some aspects, 'there is still a piece of evidence that leads us to suspect the Kepler-438 event could be an instrumental radio frequency interference signal,' the researchers concluded.

The SETI project lead at FAST, Zhang, told the state-run Science and Technology Daily on Tuesday that FAST has launched additional observations to investigate Kepler-438 in more detail.

It may take a long time to prove it either way, but even if the signal turns out to be some noise, it will still provide useful insights for our future SETI research, he said.