SETI astronomers have eavesdropped on an alien star system thought to contain two “habitable” worlds in the hope of hearing a radio transmission from an extraterrestrial intelligence.

Sadly, there appears to be no chatty aliens living around the red dwarf star Gliese 581.

In results announced last week by Australian SETI astronomers, of the International Centre for Radio Astronomy Research at Curtin University in Perth, Gliese 581 was precisely targeted by Australian Long Baseline Array using three radio telescope facilities across Australia. This is the first time the technique of very long baseline interferometry (VLBI) has been used to target a specific star in the hunt for extraterrestrials, so although it didn’t turn up any aliens, it is a proof of concept that may prove invaluable for future SETI projects.

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The Gliese 581 system, located 20 light-years from Earth, has been the focus for intense exoplanetary studies. It is thought to play host to at least six exoplanets, two of which orbit the star within the star’s “habitable zone.” This zone is the distance from a star where it’s not too hot and not too cold for water to exist in a liquid state on a hypothetical rocky world’s surface.

As this is a red dwarf star, it is smaller, dimmer and therefore cooler than our sun. As a result, the Gliese 581 habitable zone is a lot more compact than our sun’s. Gliese 581d — a “super-Earth” with a mass seven times that of our planet — skirts the outermost edge of the Gliese 581 habitable zone and has an orbital period of only 67 days. Gleise 581g on the other hand is thought to be around three-times the mass of Earth and orbits right in the middle of the star’s habitable zone. Its orbital period has been clocked at 37 days. (It is worth noting, however, the very existence of Gliese 581g has been called into question.)

Naturally, the mere hint of these “habitable” worlds in Gliese 581 has caused some excitement — they could host the perfect conditions for life (as we know it) to thrive. If there’s life, then perhaps it evolved to support intelligence; if there’s intelligence, then perhaps it has gone through a similar “radio transmitting” phase as us.

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The Search for Extraterrestrial Intelligence (SETI) has a few tricks up its sleeve to look for these hypothetical intelligent radio-transmitting aliens. Firstly, space is vast, and there are a lot of stars out there, so (until recently) the most effective method for searching for a SETI signal has been to “cast the net as wide as possible.” Using radio telescopes, surveys have scanned the sky. If an interesting signal is detected in the survey, astronomers can hone in on it for further study.

But with the deluge of data spewing from exoplanet-hunting missions — such as the Kepler space telescope, whose specialty is to seek out small potentially “Earth-like” exoplanets — SETI has another weapon in its alien-hunting armory. When exoplanets are identified (particularly ones that orbit within the habitable zones of their stars), why survey the entire sky when you can just zoom in on one interesting candidate?

This is the rationale behind focusing on Gliese 581 using the Australian Long Baseline Array — a combination of three radio antennae: the 22-meter Mopra Telescope, Parkes Observatory and the Australia Telescope Compact Array (ATCA). Signals from the three locations were combined, making them act as one huge radio telescope, allowing astronomers access to extremely focused and precise observations of a single star (with known exoplanets) while minimizing any radio interference from terrestrial sources. This method of searching for alien signals is known as “directed SETI.”

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“It’s like they’re looking at the sky through a 6-foot-long cocktail straw — a tiny bit of the sky, so they’re only sensitive to signals that are coming from right around that star system,” SETI astronomer Seth Shostak told the BBC.

After 8 hours of surveying Gliese 581, “a total of 222 potential SETI signals were detected and by using automated data analysis techniques, were ruled out as originating from the Gliese 581 system,” say the researchers in a paper submitted to the arXiv preprint service and accepted for publication in The Astrophysical Journal. Although this is a null result, it is just the first of potentially millions of candidate star systems up for directed SETI study.

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A recent estimate made by the Kepler space telescope puts 50 billion exoplanets in our the Milky Way — 500 million of which likely exist within their star’s habitable zones — and astronomers of the Probing Lensing Anomalies NETwork (PLANET) have recently announced their estimate of 100 billion exoplanets in the Milky Way.

So, although Gliese 581 may be “radio quiet,” there are plenty more exoplanetary candidates astronomers using this VLBI method to zoom-in on.

What’s more, a brand new radio telescope system called the Square Kilometer Array (or SKA) has just been given approval for construction in South Africa, Australia and New Zealand. One of SKA’s projects will be to use its incredibly powerful VLBI capability to hunt for SETI signals, so this Gliese 581 study will no doubt enrich future directed searches for intelligent extraterrestrials.

Images: Top: An artist’s impression of a multi-planetary system surrounding a red dwarf star (ESO). Middle: The radio antennae of the Australia Telescope Compact Array (ATCA), one of the facilities used in the Australian VLBI system. (CSIRO Australia)