'No signal' from targeted ET hunt

The hunt for other intelligent civilisations has a new technique in its arsenal, but its first use has turned up no signs of alien broadcasts.

Australian astronomers used "very long baseline interferometry" to examine Gliese 581, a star known to host planets in its "habitable zone".

The hunt for aliens is fundamentally a vast numbers game, so the team's result should come as no surprise.

Their report, posted online, will be published in the Astronomical Journal.

In recent years, interest in such targeted searches has begun to surge as the hunt for planets outside the Solar System continues to find them at every turn.

Astronomers currently estimate that every star in the night sky hosts, on average, 1.6 planets - implying that there are billions of planets out there yet to be confirmed.

But a number of stars have already been identified as playing host to rocky planets at a distance not too hot and not too cold for liquid water - the first proxy for amenability to life.

Gliese 581, a red dwarf star about 20 light-years away, is a particularly interesting candidate for the Search for Extraterrestrial Intelligence, or Seti.

It has six planets, two of which are "super-Earths" likely to be in this habitable zone.

So astronomers at Curtin University's International Centre for Radio Astronomy Research in Australia, put one of radio astronomy's highest-resolution techniques to work, listening in to the star system.

Very long baseline interferometry (VLBI) is the process of using several or many telescopes that are distant from one another, carefully combining their signals to make them effectively act as one large telescope, peering intently at a tiny portion of the sky.

The team trained the Australian Long Baseline Array onto Gliese 581 for eight hours, listening in on a range of radio frequencies.

The result was radio silence - but the team used their experience to validate VLBI as a technique particularly suited to this kind of targeted search.

Seth Shostak, principal astronomer at the Seti Institute in the US, said that the approach's strength lies in the fraction of the sky it examines.

"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," he told BBC News.

That is useful not only for getting a high-resolution view, but for excluding the signals from Earthly technologies that plague Seti efforts.

"Figuring out 'is this ET or AT&T?' isn't always easy, and VLBI gives you a good way of discriminating, because if you find something from that tiny, tiny dot on the sky you can say that's not one of our satellites," Dr Shostak said.

He added that the team's negative result was not disheartening, because the odds have it that the hunt for aliens, if it is ever to find them, will require thousands or millions of observations of this kind.

"Consider the fact that you could've looked at the Earth for four billion years with radio antennas - here was a planet that's clearly in the habitable zone, has liquid oceans, and has an atmosphere - and yet unless you had looked in the last 70 years and were close enough, you wouldn't have found any intelligent life," he said.

"The fact that we look at one star system and don't find a signal doesn't tell you that there's no intelligent life."