The latest hunt for alien signals in the TRAPPIST-1 planetary system has tested a new strategy that will allow astronomers to conduct a more efficient, targeted search for technological extraterrestrial life in the future.
TRAPPIST-1 is a multi-planet system approximately 40.7 light-years away. The seven rocky worlds, some of which are in the habitable zone - the zone around a star where it is not too hot or too cold for a planet to host liquid water - are all so tightly packed that they pass by their star every few days. The number of planets and their relative proximity to us make the TRAPPIST-1 system a tempting target for the search for extraterrestrial intelligence (SETI).
Although this latest search - in which the Allen Telescope Array of radio telescopes in California spent a total of 28 hours listening for TRAPPIST-1 - did not detect extraterrestrial signals, 'the aim of the study was to demonstrate a more efficient search strategy, involving "It took advantage of the natural orbital configuration of an edge-on multi-planet system to our advantage," Nicholas Tusay, a graduate student at Penn State University, told Space.com.
Traditionally, SETI has scanned the sky for powerful signals aimed at us. However, after decades of finding nothing, SETI researchers are increasingly considering other strategies. These include looking for radio leaks: incidental transmissions that are not intended for us, but that can leak from a planetary system. Such transmissions can range from communications and spacecraft emissions to radar or even the equivalent of alien TV. However, because this incidental leakage would not be transmitted with the intention of being heard light-years away, it would likely have a much lower power than intentional signals.
The chance of discovering such a leak by chance would therefore be small, so we need strategies that can improve the chances.
To this end, Tusay led the observations of TRAPPIST-1, which took advantage of a phenomenon called planet-planet occultations (PPOs). Occultation occurs when an object in the sky appears to move in front of another object. As the seven TRAPPIST-1 planets orbit their star in a plane almost perfectly sideways to us, we can witness many PPOs, with the two planets involved in the PPO and our detectors essentially all in a straight line to stand.
Related: How AI helps us search the universe for alien technosignaturesNow suppose that transmissions from the planet being eclipsed are directed to the planet performing the occultation. These transmissions could be communications similar to those of NASA's Deep Space Network (DSN), in which large radio transmitters in Canberra, Madrid and California maintain constant contact with our fleet of interplanetary spacecraft. Likewise, during a PPO, when two planets and ourselves are aligned, it is possible that we can pick up leakage from radio transmissions between the two planets from the alien equivalent of the Deep Space Network. That's what this latest study from TRAPPIST-1 was looking for.
"TRAPPIST-1 is the ideal laboratory because it has planets that move almost perfectly sideways, and because it is so close that we have enough sensitivity to detect certain signals," says Tusay.
No signals have been detected, but we don't have to be gloomy.
That's because the Allen Telescope Array is only sensitive enough to detect interplanetary transmissions on TRAPPIST-1 emitted at a power equivalent to that of an Arecibo-sized transmitter. Before it collapsed in 2020, the Arecibo radio telescope was a 305-meter (1,000-foot) dish. However, a powerful transmitter like Arecibo would be "overkill" for interplanetary communications, Tusay said. The DSN's telescopes are smaller in size with lower effective power - too low for the Allen Telescope Array to detect. However, when it begins scientific activities towards the end of this decade, the Square kilometer array in South Africa and Australia should have the sensitivity to detect DSN level transmissions during PPO events.
The Allen Telescope Array experiment has now shown that this PPO method is feasible. The ATA observed seven PPO events during the 28 hours it stared at TRAPPIST-1 in 2022. In total, it detected 25 million radio signals during that time.
'Most of it radio frequency interference (RFI) from our own communications, so we had to filter that out," Tusay said.
Radio frequency interference (RFI) is the terrestrial background of radio signals Earth from mobile phones to airport radar. To more easily remove RFI from the observations, Tusay developed something called the NBeamAnalysis pipeline. It is computer code that can distinguish signals coming only from the target, in this case TRAPPIST-1, from RFI seen in other directions in the telescope's field of view. By doing this, the code was able to reduce the 25 million signals to just 2,264 signals that required further attention from a human.
"Instead of looking at tens of millions of hits by eye, I only have to look at a few thousand, and most of them are still obvious to the human eye as RFI," Tusay said.
Ultimately, all signals detected during the TRAPPIST-1 observations were RFI, but there are reasons why the Allen Telescope Array should keep watching. While we can only guess at the nature of an alien communications system and how often non-terrestrials would communicate with neighboring planets, comparing their signals to our own Deep Space Network is a starting point. Tusay's team estimates that the DSN ships to Mars about a third of the time, meaning that aliens would have to watch an average of three PPO events on Earth and Mars to see that we are sending a signal to one of our spacecraft around the Red Planet. If aliens follow a similar cadence to TRAPPIST-1, we'll need to watch at least three PPO events from each combination of planets to have the best chance of spotting them.
This raises a question: could aliens look for PPOs from planets in our own? solar system? That would happen less often than in the TRAPPIST-1 system, where the planets are so close to their star that they orbit Earth within a few days. Conversely, PPOs of Earth and Mars would occur approximately once every two years. To see a PPO of Earth and Mars, aliens would also have to be on a planet orbiting a star that is in the ecliptic in the sky, because the ecliptic is the plane of our solar system, and only by seeing this plane edge -she would see any transits or occultations.
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Nevertheless, "I know that the search for evidence that our own DSN transmissions have been picked up is an active area of research by other SETI scientists," Tusay said. "Personally, I think that search strategy has advantages."
In the meantime, we'll have to keep listening to the skies - and thanks to these new observations from TRAPPIST-1, we now at least have a better idea of the best times to listen.
The findings are described in a paper accepted for publication in The Astronomical Journal, and there is a pre-printing available on arXiv.
