Radio signal-hunting astronomers find no alien life near Milky Way stars

After a hunt for Milky Way radio signals came up empty, the team of astronomers behind it has estimated that fewer than one in a million stars in the galaxy could have advanced civilisations residing on their orbiting planets.

The team, which used the Green Bank Telescope in West Virginia to look for signals with a frequency range of between 1.1 and 1.9 GHz (the mobile phone and TV band range on Earth), has published a paper online revealing its findings -- or lack thereof -- and prediction.
The project was headed up by Jill Tarter of the SETI (search for extraterrestrial intelligence) Institute, who has spent the majority of her career in astronomy seeking signs of life in the Universe. With the advent of the planet-hunting Kepler telescope, she and her University of California, Berkeley team set about targeting 86 star systems found by it -- the stars were chosen for either hosting planets within the habitable zone, having five or more planets in their orbit or super-Earths with long orbits. By the time the research was launched (from February to April 2011), Kepler had already idenitified 1,235 exoplanets.
Each star was targeted for about five minutes, to gather enough radio emissions. The team then looked for evidence of narrow bandwidths in the data of no more than 5 Hz -- at such intensity, signals can only be artificially engineered (at least, that is, as far we know). Sadly, no such signal was found and thus the team states in the paper that it "identified no evidence of advanced technology indicative of intelligent life".
"We didn't find ET, but we were able to use this statistical sample to, for the first time, put rather explicit limits on the presence of intelligent civilisations transmitting in the radio band where we searched," UC Berkeley astronomer Andrew Siemion said in a statement.
There are plenty of reasons why existing signals may have not been picked up, however. For instance, the telescope was only powerful enough to pick up signals directed straight at it from the 1,000-light-year distance most of the planets were located. In the future, the team would like to try out much stronger telescopes, for instance the Square Kilometre Array, due to be finished in 2024. The array will, predicts Tarter and her team, "be perhaps 100 times more sensitive than the GBT" and it therefore may not matter whether the signal is directed straight at Earth. If they were to use SKA, and a distant civilisation had the power to send out stronger signals, life would be relatively easier to spot.
Considering Kepler's only been up and running since 2009 and it's already spotted 2,740 exoplanets to date, there's plenty of scope for future research and progress in the field. Even now, teams are making adjustments to certainties Tarter and her team took for granted while carrying out their research, For instance, the habitable zone at which life can exist has recently been amended. According to a study from Penn State University, based on information from two atmospheric databases that detail absorption rates of water and carbon dioxide, the so-called Goldilocks zone at which a planet could host liquid water was slightly off (in our solar system, it's shifted from being between 0.95 and 1.67 astronomical units (the distance from the Sun) to between 0.99 and 1.7 astronomical units).
"Right now I see it as a significant change," commented Abel Mendez of the University of Puerto Rico, who was not involved in the study. "Many of those planets that we believe were inside are now outside. But on the other side, it extends the habitable zone's outer edge, so a few planets that are farther away might fall inside the habitable zone now."
It means some of the stars Tarter and her team focused on may have potentially hosted planets outside of the habitable zone.
To widen the parameters of its study, the team is hoping to look into planetary pairs that might be communicating with one another, and that routinely line up with Earth. Considering they've already set their predictions at one in a million, what the likelihood of finding two planets having a galactic chat is remains to be seen. On the bright side, such a signal would have to be narrowly beamed and powerful, predicts the team, so would be easier to spot.
The SETI team does admit, however, that when it really comes down to it we have no idea how a distant civilisation would function and communicate -- it's all based on educated predictions.
"In particular, we can offer no argument that an advanced, intelligent civilisation necessarily produces narrow-band radio emission, either intentional or otherwise," reads the paper's conclusion. "Thus we are probing only a potential subset of such civilisations, where the size of the subset is difficult to estimate." Earthlings, for instance, have been producing those kinds of signals for only a mere fraction of their existence.
As things like the semiconductor business continue to boom, however, progress is advancing, they say.
"Within the next decade, we will have the ability to examine significantly larger portions of the electromagnetic spectrum, including instantaneous analysis of the entire 10 GHz of the terrestrial microwave window. In addition to radio searches, new technology will extend SETI into regions of the electromagnetic spectrum never before observed with high sensitivity. Extending searches to encompass much larger classes of signals is crucial to producing robust and meaningful limits."
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