K2-18b is a so-called Habitable Zone planet located 124 light-years away from us. In terms of mass, the planet is somewhere between Earth and a small gas giant. Researchers are interested in whether extraterrestrial life could exist on it.
Primary goal for SETI
If you have been following exoplanet research over the past few years, you have probably heard of K2-18b. Located 124 light-years away in the constellation Leo, it has attracted a lot of attention because it is right in the habitable zone of its red dwarf star, and measurements from the James Webb Space Telescope show that its atmosphere is rich in carbon dioxide and methane. It is one of the leading candidates for the title of “hydrogen world” — a world where a dense atmosphere rich in hydrogen covers a global ocean of liquid water.
This is such an interesting target for researchers from the Search for Extraterrestrial Intelligence (SETI) project that they have directed two of the world’s most powerful radio telescopes to observe the K2-18b system. A recent article available on the arXiv preprint server shows that, despite millions of potential matches, this planet is unlikely to be sending out artificial narrowband radio signals equivalent to our level of technology.
Listening with radio telescopes
To collect the data needed for processing, both the Karl G. Jansky Very Large Array (VLA) in New Mexico and the MeerKAT radio telescope in South Africa were used. These are the two most powerful radio telescopes on the planet, and their coordination within the observation campaign is an extremely rare occurrence.
But physical equipment was not the only important factor for this work. The “data transfer channel,” as astronomers call the software filters and logic applied after data collection, is no less important, especially in radio astronomy. Signals from Earth are the source of the vast majority of radio signals received by these telescopes, and modern filtering algorithms, such as the Commensal Open-Source Multi-Mode Interferometer Cluster system used by the VLA, and the Breakthrough Listen User Supplied Equipment (BLUSE) system used by MeerKAT, are important elements of any modern radio astronomy program.
Radio signal filtering system
However, the logic behind such filtering remains the responsibility of the people involved in the process, and the article describes five different restrictions they impose on the data for selecting potential alien technosignatures. The first step was to mask radio frequency interference — essentially, they removed all data from signals falling within frequency ranges known to be heavily contaminated by terrestrial interference. If aliens communicated through these channels, we would have to use some other method — such as a radio telescope on the far side of the Moon — to hear them.
Doppler effects, such as the changing sound of an ambulance siren as it approaches or passes you, are even more noticeable when the signal travels between planets. Any signal with virtually no Doppler effect was immediately rejected, as it could only have originated from Earth. Perhaps the most controversial logical filtering choice was to exclude all signals with a signal-to-noise ratio of less than 10 or greater than 100. Although this allowed for the exclusion of extremely weak false positives, as well as strong instrumental data artifacts typically observed in only one antenna, it could also exclude.
Another filtering technique is the use of multi-beam analysis. In this case, the telescopes formed coherent beams across the sky, one of which was directed directly at K2-18b, and the other at a different location. In such cases, the signal coming from the exoplanet would only appear in the beam directed directly at it, while terrestrial interference penetrates several beams simultaneously. The final check, which was not necessary due to the timing of the study, is transit filtering. Any signal coming from K2-18b should disappear when the planet passes in front of its parent star, but since no such “secondary transit” occurred during the observation, filtering was not necessary.
Positive test result in any case
In short, despite millions of potential signals throughout the observation period, none of them passed these filters. No unambiguous technosignatures were detected in the narrowband radio spectrum from K2-18b. Although this may seem disappointing, it is precisely this kind of thing that science needs in order to progress. By carefully scanning the planet and finding nothing, they can establish the “upper limits” of the transmitter’s power from this system — in terms of power, it would be equivalent to the destroyed Arecibo radar in Puerto Rico. If there is civilization there, it certainly does not scream at us from anything greater than this level of radio telescope.
However, perhaps the most important result is the confirmation of the concept of their automated filtering system. Processing millions of signals detected by two telescopes manually would be virtually impossible. So, when even larger radio telescopes such as the Square Kilometer Array appear, these technologies will be ready to help other research projects make sense of the mass of data collected. Although planet K2-18b may be quiet today, we will continue to improve our ability to listen if it ever begins to communicate with us.
According to phys.org
