A new European search for extraterrestrial radio signals at low, uncharted frequencies is underway, having already listened to over 1.6 million star systems.
SETI, the search for extraterrestrial intelligence, has traditionally focused on radio frequencies higher than a gigahertz, such as the hydrogen-line frequency at 1.42 GHz. SETI astronomers tend to shy away from lower frequencies because Earth's atmosphere renders observations noisy.
However, Europe's Low Frequency Array, or LOFAR for short, is specially designed to conduct radio astronomy at these very frequencies.
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LOFAR is an array of radio antennas that span hundreds of kilometers across Europe, centered in the Netherlands but with additional stations in France, Germany, Ireland, Latvia, Poland, Sweden and the United Kingdom. The stations incorporate two types of antenna: low band antennas that operate between 10 and 90 MHz, and high band antennas that listen to the universe between 100 and 250 MHz.
In conjunction with the Breakthrough Listen SETI project, the LOFAR stations in Ireland and Sweden have been used in conjunction with one another in the first part of Breakthrough Listen's first-ever low-frequency search.
This search used the high band antennas to listen for radio signals at frequencies of between 110 and 190 MHz. Primarily, the search is looking for leakage from high-power transmitters, such as planetary radar or communications with spacecraft. The search encompassed 1,631,198 target star systems identified by NASA's Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency's Gaia astrometric probe.
By using multiple sites in Ireland and Sweden, astronomers were able to negate the effects of radio-frequency interference and quickly rule out any false positives. For example, if an anomalous signal were only spotted by one station and not the others, it would be local interference. Only a signal coming from space could be detected by all the stations.
No narrowband radio signals with a distinctive frequency drift caused by the orbital motion of an exoplanet hosting a transmitter beaming out signals with a power of at least tens of millions of watts were detected. However, the low-frequency search is only just beginning, and improvements in coming years will increase its sensitivity.
"LOFAR is soon to undergo a staged series of upgrades across all stations in the array across Europe, which will allow an even broader SETI at ranges of 15-240MHz," said graduate student Owen Johnson of Trinity College Dublin, who is the lead author of a new paper describing the results, in a statement.
Among these upgrades will be two new LOFAR stations in Bulgaria and Italy. Computing software and artificial-intelligence algorithms will also speed up the analysis of the results.
"We have billions of star systems to explore and will be relying on some machine-learning techniques to sift through the immense volume of data," said Johnson. "That in itself is interesting — it would be fairly ironic if humankind discovered alien life by using artificial intelligence."
The first results from the LOFAR SETI search were published on Oct. 24 in The Astronomical Journal.
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