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Jill Tarter
SETI pioneer Jill Tarter pays a visit to the Allen Telescope Array in California, one of the prime sites for the search for extraterrestrial intelligence. (Credit: SETI Institute)

It’s a question that goes back decades: If other civilizations have arisen beyond Earth over the course of billions of years, why haven’t we heard from them? Two kinds of answers have recently come into the spotlight – one kind that’s disheartening, and another kind that’s challenging.

First, the bad news: Researchers from the Australian National University say climate change could have killed off E.T. and his ilk.

In a paper published last month in the journal Astrobiology, Aditya Chopra and Charles Lineweaver suggest that habitable Earthlike planets eventually fall prey either to runaway global warming, as in the case of Venus; or runaway global cooling, as in the case of Mars.

Their model proposes that if life ever does arise on an alien world, it would go extinct in most cases after just 1.5 billion years of the planet’s existence, without getting past the microbial stage.

Less than 10 percent of the habitable planets, including Earth, would pass through a “Gaian bottleneck.” That means life progresses to the point at which it controls its own planetary environment. On Earth, for example, plant and animal forms of life are in a balance to keep water, carbon dioxide and oxygen at life-sustaining levels.

Even in those cases, the Gaian bottleneck model shows life fading away after about 6 billion years, due to an increase in stellar luminosity for sunlike stars. (We’re at the 4.6 billion-year point here on Earth.)

Aditya Chopra
Astrobiologist Aditya Chopra says life forms on other planets would commonly die out due to runaway heating or cooling on their home planets. (Credit: Stuart Hay / ANU)

Chopra and Lineweaver say the Gaian bottleneck could provide one of the answers to Fermi’s Paradox – that is, if the universe is conducive to the rise of life, “where is everybody?” It could also explain why the search for extraterrestrial intelligence, or SETI, has come up short after nearly 60 years of looking.

“If it takes several billion years to develop radio telescopes, then the Gaian bottleneck ensures that the vast majority of life in the universe is either young and microbial, or extinct,” the researchers write.

Before you get too depressed at that thought, consider the perspective of Jill Tarter, a pioneer in SETI research who helped inspire the character of Ellie Arroway in the Carl Sagan book (and Jodie Foster movie) “Contact.” Tarter holds the Bernard Oliver Chair at the SETI Institute’s Center for SETI Research, and recently talked with TechCrunch about the search for alien signals.

The way Tarter sees it, the search has only just begun. Here’s the way she explained it in a TED talk: “All of the concerted SETI efforts over the last 40-some years are equivalent to scooping a single glass of water from the oceans, and no one would decide that the ocean was without fish on the basis of one glass of water.”

More fundamentally, the search is limited by our imagination. We’re still merely looking for the kinds of signals that we would send out if we were a super-advanced civilization – that is, radio and laser semaphore messages.

“We may not have invented the right way to do this yet,” Tarter told TechCrunch.

That meshes with a comment that Tarter made in a video that’s part of WeTransfer’s Creative Class series: “How do you think about finding the unknown when you don’t know how to find it?”

The search for unorthodox ways to look for alien life is likely to lead down some weird trails – for example, the questions about “alien megastructures” that were recently raised by observations of a strangely dimming star. But for Tarter, and others in the SETI search, the trip is worth it.

“I think SETI is something that gives us this extraordinary opportunity to see ourselves in a different way and change our perspective,” she said. “And if we could change the way people around the world thought about who they are and how they fit in, and that they are part of a really big, big, big, big picture, I think we could do a lot of creative things in terms of solving the problems and reducing the tensions among nations and among individuals.”

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