A search for colossal feats of alien engineering called 'Dyson spheres' has so far found no convincing candidates within 1000 light years of Earth. But some say the prospects for finding the hypothetical structures, which could cocoon stars in order to collect solar energy for power-hungry aliens, may be getting brighter.

The search for extraterrestrial intelligence is focused largely on detecting signals sent from afar. But in 1960, physicist Freeman Dyson proposed a way to directly search for artifacts of alien civilisations.

Dyson envisioned that population pressure and the demand for energy would drive civilisations to dismantle planets and use the debris to surround a star, creating a massive solar collector.

A number of Dyson sphere structures have been proposed, including a solid, rotating ring and a spherical shell of debris. These structures might be habitable themselves, or they might be used as remote solar power collectors.

The structures would partially or fully block the star's visible and ultraviolet light, but they would still be detectable. A Dyson sphere or ring would be warmed by the star's energy and would radiate infrared light that could be detected from Earth.

Some researchers have previously searched for signs of the spheres in data from the Infrared Astronomical Satellite (IRAS), which launched in 1983 and made an infrared map of the entire sky.

Hundreds of candidates

IRAS took data in different modes, sometimes looking at its sources through a handful of colour filters, and sometimes looking at sources with its spectrograph, which carefully studied a wide range of colours.

The previous searches focused on sources that had been studied with the colour filters. They looked for objects that radiate most of their light at relatively long infrared wavelengths. That would be the case if a Dyson sphere located at about the Earth's orbital distance from its star absorbed the energy emitted by a Sun-like star and then re-emitted it as heat.

But with only a few colours available to scrutinise for the temperature signature of a Dyson sphere, the previous studies could not whittle down the list of 250,000 IRAS sources to a few good candidates.

Now, Richard Carrigan, a retired physicist who had worked at the Fermi National Accelerator Laboratory in Batavia, Illinois, has combed through data from the probe's spectrograph, which provides a more detailed portrait of the source's light.

Whittling down

Beginning with more than 10,000 infrared spectra taken by IRAS, Carrigan identified only 17 possible "spheres", four of which seem most promising.

Unfortunately, all the objects have features that could just as easily be explained by clouds of hydrogen gas, dust engulfing ancient stars, or even asteroids in our own solar system, Carrigan told New Scientist.

"There are very few candidates that come close to fitting the bill," Carrigan says.

"Many different kinds of astrophysical objects could masquerade as Dyson spheres," says infrared astronomer Charles Beichman of Caltech. "I think the search . . . is looking for a needle in a field of haystacks, when you're not even sure there's a needle there. But he's done a very nice job of working through the available data."

Carrigan's approach is good at whittling down sphere candidates from many sources, agrees astronomer Dan Werthimer of the University of California, Berkeley, and chief scientist of SETI@Home, a project that harnesses the idle time of people's computers to search for possible extraterrestrial radio signals.

"It gets it down to a manageable number of stars you can follow up on," Werthimer told New Scientist. "That's been a big problem with these Dyson sphere searches."

More precise

The 17 sphere candidates Carrigan has identified have been added to SETI's list of interesting objects, to be investigated for radio and laser signals. "If we're lucky, maybe one of these things will pan out and be the first evidence of an alien civilisation," Werthimer says.

But Werthimer adds more promising evidence for Dyson spheres could come from NASA's Spitzer Space Telescope.

IRAS was not precise enough to identify individual stars, particularly in crowded parts of the sky, such as the plane of the Milky Way. "You might have 10 or 20 sources within the IRAS beam, but what you're getting is the sum of all that flux, not the signature of a single Dyson sphere," says astronomer Ed Churchwell of the University of Wisconsin.

Spitzer's GLIMPSE survey may offer better prospects for Dyson structure searches. The all-sky infrared map contains more than 100 million objects and has some 60 times IRAS's resolution, Churchwell told New Scientist. A final version of the survey's data may be available as early as the end of 2008.