K2-18b, the star exoplanet of recent years, has just passed a very particular test: astronomers literally listened to find out if an extraterrestrial civilization was broadcasting artificial radio signals there. Located 124 light years away in the constellation Leo, this planet is in the habitable zone of its red dwarf star and appears to be surrounded by an atmosphere rich in CO₂ and methane, making it a prime candidate for status as a “Hycean” world, covered by a vast liquid ocean under a thick atmosphere of hydrogen. Intriguing for exobiologists, it is just as intriguing for hunters of technosignatures, explains Universe Today.
To try to capture possible signals, the researchers mobilized two of the most powerful radio telescopes in the world: the Karl G. Jansky Very Large Array (VLA), in New Mexico, and MeerKAT, in South Africa. Such a coordinated observation campaign is extremely rare, as these instruments are in great demand. The idea: to scan the region of the sky where K2-18b is located for long hours to find narrow-band radio signals – the type of signals that we produce ourselves with our radars and large transmitters.
But in this type of research, equipment is not everything. The crucial step then comes with the software filters which, after the observation, sift through millions of potential signals to eliminate background noise. Most of the signals picked up by these antennas are actually terrestrial and come from satellites, radars, and various communications. Astronomers have therefore relied on sophisticated algorithms, capable of identifying these intruders and eliminating them, even before hoping to come across a trace from a distant planet.
The researchers thus detail a series of logical constraints applied to the data to isolate possible technosignatures. First step: mask the frequency bands polluted by known interference of human origin, a major cleaning which has the side effect of excluding the possibility that extraterrestrials are using precisely these “bad” frequencies. Next, they eliminate any signals that show no Doppler effect—that slight frequency drift due to the relative movements of the planets and antennas, comparable to the change in the sound of a passing ambulance siren.
Tracking down parasites
One of the most questionable, but necessary, choices concerns the signal-to-noise ratio. The scientists decided to discard all signals whose signal-to-noise ratio was less than 10 or greater than 100. Below that, too many weak false positives; above, a high risk of artifacts often seen by a single antenna and not the full array. This pragmatic cut could certainly cause a discreet extraterrestrial signal to be overlooked, but without it, the mass of “false” signals would make the analysis completely unmanageable.
Result of this battery of filters: despite millions of raw signals recorded, none has passed all the sorting stages. No narrow-band radio technosignatures, typical of technology comparable to ours, have been detected in the K2-18b system. Disappointing for science fiction lovers? Not really for researchers, who point out that this type of “non-result” is essential since it allows us to establish quantitative limits on what could exist in our universe.
In this case, all that the team shows with this study is that if a civilization is on K2-18b, it does not use, towards Earth, a radio transmitter more powerful than what was produced, for example, by the large Arecibo radar, now abandoned. In other words, if inhabitants of this Hycean world exist, they are not “yelling” in our direction with massive radio resources comparable to ours. They could be silent, use other technologies, or simply be less evolved than us.
The other major conclusion of the work is methodological: this campaign constitutes a proof of concept for automatic filtering systems. Manually sifting through the millions of signals detected by two large radio telescopes would be impossible. The tools developed here show that it is possible to process this flow of data in a robust manner, by systematically rejecting terrestrial noise pollution without drowning out a possible interesting anomaly.
This is an essential step before the arrival of the next generation of radio observatories, such as the Square Kilometer Array, which will generate even more data. The techniques tested on K2-18b will then serve as the basis for much more ambitious surveys of the sky, increasing the chances of one day coming across a real technosignature.
