A telescope’s angular resolution is its ability to distinguish small details of a distant object. The Hubble Space telescope, for example, has an angular resolution of about 100 milliarcseconds.
That’s good but by no means the best. In fact, the telescopes with the highest angular resolutions are interferometric radio telescopes, made up of several dishes spread over thousands of kilometres.
Known as very long baseline interferometers (VLBIs), the biggest boast an angular resolution some two orders of magnitude better than Hubble.
So what to point them at? Today, Hayden Rampadarath and pals at the International Centre for Radio Astronomy Research at Curtin University in Australia say they’ve pointed their interferometric radio telescope at Gliese 581, a red dwarf star some 20 light years from here.
What makes Gliese 581 interesting is its planets, which include two superEarths that probably sit on the edge of their habitable zone.
That makes them good candidates for life. And if this life is anything like our own, it may already be broadcasting at radio frequencies that we can tune in to.
Although VLBI has extraordinary angular resolution, it has never been used to look for signs of extraterrestrial intelligence. So this is an important proof-of-principle step.
The Australian instrument, known as the Australian Long Baseline Array, consists of three radio telescopes a few hundred kilometres apart, which gives them an angular resolution that is about the same as Hubble’s.
Rampadarath and pals pointed it at Gliese 581 for a total of 8 hours in June 2007, tuning into frequencies close to 1500 megahertz. Why they’ve waited so long to publish their result, they don’t say but their paper has now been accepted for publication in The Astronomical Journal.
What they found is interesting. VLBI techniques turn out to be useful for SETI searches because they automatically exclude many terrestrial sources of interference that might otherwise look like SETI signals. That’s because the same signals have to show up at all the telescopes several hundred kilometres apart.
In total, Rampadarath and co found 222 candidate SETI signals. However, they were able to exclude all of these relatively easily using automated analysis techniques, which have become increasingly sophisticated in recent years. (That’s partly because of projects such as SETI@Home which has found billions of interesting signals, all of which have turned out to be false alarms.)
The false alarms picked up by the Australian Long Baseline Array probably came form Earth orbiting satellites, say the team.
Of course, this doesn’t exclude the possibility of intelligent life in the Giese 581 system or even exclude the possibility that these ETs might use radio signals to communicate.
Instead, it places limits on the strength of these signals and not particularly onerous ones at that. Rampadarath and pals say their instrument would have picked up a broadcast with a power output of at least 7 megaWatts per hertz.
To put that in context, on the slim chance that Gliese inhabitants had been broadcasting directly to Earth using an Arecibo-style dish, Rampadarath and co would have easily picked up the signal. (Arecibo is a 300 metre radio telescope in Puerto Rico).
On the other hand, the ordinary radio transmissions like those we continually broadcast into space, would have been far too weak to be picked up by the Australian team.
That’s not to say that this kind of observation won’t be possible in future. The Australian array is by no means the biggest of most sensitive instrument available today.
What’s more, astronomers are planning a new VLBI telescope called the Square Kilometre Array which will have the sensitivity to pick up broadcasts of a few kiloWatts per Hertz from 20 light years away.
There are no shortage of targets. At the last count, astronomers had found around exoplanets that sit in their habitable zones (meaning they’re warm enough for liquid water). These places are of intense interest.
Time on VLBI telescopes is precious and difficult to come by. But the prize here is of almost incalculable value–the discovery of intelligent life beyond the Solar System.
So it wouldn’t be a complete surprise if radio astronomers found ways to hunt more often for radio broadcasts from new and exciting exoplanets.
Ref: arxiv.org/abs/1205.6466 :The First Very Long Baseline Interferometric SETI Experiment