The discovery of an ever-growing number of potentially habitable exoplanets brings an extra spiciness to the search for extraterrestrial intelligence. For the first time, astronomers can direct the search towards these likely planets rather than aiming in hope towards the stars.

Today, Jill Tarter, from the SETI Institute and of Contact fame, along with a group of buddies, reveal the results of their first directed search, carried out between February and April 2011.

These guys pointed the Green Bank Telescope in West Virginia at 86 stars hosting exoplanets discovered by the Kepler space telescope. They chose their targets because they had exoplanets in the Goldilocks zone, had five or more exoplanets or had super Earths with relatively long orbits.

Tarter and co looked at signals in the 1-2 GHz range, the region used by terrestrial mobile and cordless phones. In particular, they hunted for signals that cover no more than 5Hz of the spectrum since there is no known natural mechanism for producing such narrow band signals. “Emission no more than a few Hz in spectral width is, as far as we know, an unmistakable indicator of engineering by an intelligent civilization,” they say.

The big challenge with these kinds of observations is to rule out the false positives generated on Earth. Tarter and co developed a technique based on the simple idea that a signal can only be interesting if it appears in the data while the telescope is pointing at the target star but not when the telescope is pointing somewhere else. “This excluded 99.96 per cent of the candidate signals,” they say.

That left 52 candidate signals which Tarter and co then studied for signs of a terrestrial origin.

Their conclusions are forthright. “No signals of extraterrestrial origin were found,” they say.

There are some important caveats, however. In particular, is the question of how strong a signal the Green Bank Telescope can pick up.

Tarter and co consider in particular the most powerful beam that humans could broadcast into space: the Arecibo Planetary Radar in Puerto Rico. They say that if such a beam were pointed towards Earth during their experiment, they would have spotted it at distances of up to 10,000 light years. Of course, the likelihood of such a happy coincidence is small.  

More advanced civilisations might have more power to play with and so be easier to see. In particular, civilisations that have harnessed all the energy from their star–so-called Kardashian Type II civilisations–ought to be easy to spot.

The results allow the team to put important limits on the likelihood of Kardashian Type II civilisations. Tarter and co say that the negative result implies that the number of these civilisations that are loud in the 1-2GHz range must less than one in a million per sun-like star.

That still leaves plenty of wiggle room. And the team points out that rapid improvements in the technology for sensing radio signals means that researchers ought to be able to tighten these limits significantly in the not too distant future.

Ref: arxiv.org/abs/1302.0845: A 1.1 to 1.9 GHz SETI Survey of the Kepler Field: I. A Search for Narrow-band Emission from Select Targets