Astronomers are planning a massive 250-foot telescope that will search for extra-terrestrial life by detecting the heat that alien civilisations may produce.

The proposed instrument — called Colossus — would measure 77 metres which is more than double the aperture of any telescope yet constructed. It is expected to cost $1 billion.

The telescope would be sensitive enough to spot cities or other signs of aliens for planets as far as 60 to 70 light-years from Earth, astronomers said.

“If we had an investor come and say ‘look, here are the resources you need’, we could have the telescope built within five years,” said Jeff Kuhn, an astronomer at the University of Hawaii’s Institute for Astronomy, who is on the proposal team.

In searches for extra-terrestrial intelligence, astronomers generally focus on seeking out beamed signals from other civilisations. In four decades of searching, nothing definitive has been found, SPACE.com reported.

There are limitations with this method, however. Perhaps the aliens might not send out signals themselves. Perhaps they broadcast in channels humans would not think of using.

Moreover, humans should perhaps be cautious about sending out signals and alerting more advanced civilisations to their presence, as physicist Stephen Hawking has said.

This is where Colossus can shine, Kuhn said. The telescope is a passive receiver that allows astronomers to seek out extra-terrestrials without alerting them to the search.

Kuhn’s team builds on a concept first proposed by physicist Freeman Dyson in the 1960s. Humans can capture only a fraction of the energy sent out by the Sun, but a more advanced civilisation would want to grab as much as possible.

Dyson suggested an extraterrestrial civilisation would surround their star with a structure — now known as a “Dyson sphere” — that would capture the energy needed and then bleed the rest off into space.

From Earth, a star that is faint optically but very strong in the infrared could be an indication of such a sphere, Dyson mused.

Kuhn’s team, rather than focusing on stars, is instead looking at the surfaces of alien planets.

“Similarly, an exoplanet that was optically dark, but thermally bright, would be evidence of extra-terrestrial civilisation,” Kuhn said.

“The biggest telescopes that we’re likely to see in the next 100 years or so will not be able to directly image cities or organised structures on the planet,” he said. Still, he added, local heat sources could be visible.

There’s no firm location yet for the telescope, but Kuhn suggested it could be built in the San Pedro Martir mountainous area of Baja California in Mexico, close to the location of one partner in the project: the National University of Mexico in Ensenada.

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