In an effort to search for intelligent extraterrestrials, astronomers have completed their first “directed” SETI search. Unfortunately, it turned up no evidence of transmitting aliens. But that’s hardly surprising.

By focusing the Green Bank radio telescope, located in West Virginia, on stars hosting (candidate) exoplanets, it is hoped that one of those star systems may also play host to a sufficiently evolved alien race capable of transmitting radio signals into space. But in a study headed by Andrew Siemion, of the University of California, Berkeley, the conclusion of this first attempt is blunt: “No signals of extraterrestrial origin were found.”

PICTURES: Which Exoplanets Could Host Life?

With the help of the Kepler space telescope, Siemion’s team was able to identify which stars in Kepler’s field of view host exoplanets with certain characteristics. By selecting star systems hosting worlds in their habitable zones, systems containing 5 or more exoplanets and super-Earths with an orbital period of over 50 days, the astronomers hope that the evolution of intelligent life may have been possible, and thus may contain a transmitting alien race.

But it’s a bit like trying to find a needle in a haystack, when you have no clue where the haystack is.

“In particular, we can offer no argument that an advanced, intelligent civilization necessarily produces narrow-band radio emission, either intentional or otherwise,” the astronomers caution in a paper submitted to the arXiv pre-print service today. “Thus we are probing only a potential subset of such civilizations, where the size of the subset is difficult to estimate.

“The search for extraterrestrial intelligence is still in its infancy, and there is much parameter space left to explore.”

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86 stars were selected for this directed search, and over a period of three months in 2011 (February to April), the Green Bank antennae detected 52 candidate narrow-band radio signals (less than 5 Hz) between 1-2 GHz. There are no known natural mechanisms for generating radio signals at such narrow frequencies. “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 added.

After careful analysis, all signals were ruled out as being anything extraterrestrial; they were identified as interference from artificial terrestrial sources.

Though this search didn’t turn up evidence for ET’s phonecall, it represents a critical step forward in the detection of intelligent civilizations beyond our solar system. Whereas previous searches for transmitting aliens depended on educated guesswork, Kepler can be used as a guide for radio telescopes to hone-in on systems known to contain small worlds with some orbital characteristics like Earth.

ANALYSIS: Super-Civilizations Might Live Off Black Holes

This non-discovery is a result in itself — the researchers can now place important limits on the likelihood of finding a sufficiently advanced alien race in the Milky Way. Generating a powerful radio signal requires a lot of energy, so the team point out that they will most likely detect a civilization capable of generating an isotropic signal (i.e. a radio transmission that is emitted in all directions). This would require the civilization to harness the total power output of their host star, making them a Kardashev type II civilization. The results from this study give a statistical likelihood of no more than on-in-a-million chance of sun-like stars playing host to such advanced civilizations.

Refining SETI searches are key to us one day detecting hypothetical extraterrestrial intelligences. The next logical step would be to probe exoplanetary atmospheres for spectroscopic traces of a biosphere, a path Kepler has started to lead us down.

Source: arXiv blog

Publication: “A 1.1 to 1.9 GHz SETI Survey of the Kepler Field: I. A Search for Narrow-band Emission from Select Targets,” arXiv:1302.0845v1 [astro-ph.GA]

Image: The Greenbank Radio Telescope. Credit: NRAO

Note: The original version of this article stated that ex-SETI chief Jill Tarter headed this research. This was not the case, Tarter was a contributing researcher. Corrections have been made to reflect this.