The modern search for extraterrestrial intelligence, known as SETI, began at radio observatories more than 50 years ago. Now scientists at the Murchison Widefield Array (MWA) in Western Australia have launched a new SETI endeavor — the first to look for signals in the low-radio-frequency regime.
Because radio waves are such an important part of human communication, many SETI experiments search for signals from alien civilizations in radio bands — typically in the 1.4–1.7 GHz frequency range. But recently, new radio observatories are being developed in the lower-frequency range, including the MWA at 80–300 MHz.
The primary goals of the MWA are to detect neutral atomic hydrogen from the Epoch of Reionization and to study the Sun, heliosphere, and Earth’s ionosphere. In July of 2014 it was undertaking a spectral line survey of the galactic plane, when a team of scientists led by Steven Tingay (ICRAR, Curtin University, Australia; National Institute for Astrophysics, Italy) realized that this data could also be used for SETI purposes.
The Hunt for CommunicationThe MWA presents a unique opportunity for SETI: it’s on an extremely radio-quiet site, it’s a high-sensitivity array operating in a unique frequency range, and it has access to the southern hemisphere and an exceptionally large field of view.
Tingay and collaborators examined observations from the MWA that consisted of spectra in the 103–133 MHz range for each part of the sky in a 400-square-degree field of view. The authors compared this field to the Kepler catalog, identifying 45 planets in 38 known planetary systems within the field. They then examined the MWA spectra at the locations of each planetary system, searching for narrowband radio signals coming from the systems.
No Civilizations Yet
No such signals were found, and Tingay and collaborators used their observations to set upper limits on the power of any isotropic emission coming from each system. As an example, they found that no signals in the 103–133 MHz range are being broadcast from the planets around GJ 667C — which is 22 light-years away — with a power greater than ~1013 W.
This limit, however, is still 1000 times more powerful than the most powerful transmission ever deliberately broadcast into space by humans: a message sent from the Arecibo observatory with an equivalent isotropic transmission of ~1010 W. This means that these observations from the MWA may not be sensitive enough to detect messages from hypothetical alien civilizations.
Proof of Future CapabilitiesTingay and collaborators are undeterred, however, arguing that the results motivate a deeper and larger search. This initial study covered a limited part of the sky at limited frequencies; the authors suggest that the next step is to perform a SETI experiment to the same depth as these observations, but over the full MWA frequency range and the full sky accessible from Western Australia. This would require a feasible ~1 month of observing time.
Longer observing time still could search even deeper, and future arrays observing in similar frequency ranges, like the Square Kilometer Array, will be even more sensitive. This study demonstrates the utility of such arrays for conducting SETI experiments in this new low-frequency band.
S. J. Tingay et al 2016 ApJ 827 L22. doi:10.3847/2041-8205/827/2/L22