Kiwi's theory: how we could find life beyond Earth

A new study has looked at our own planet's history for tell-tale atmospheric bio-signatures that could be used to find life on others. Photo / 123RF

A Kiwi researcher has proposed a new way to help answer one of the biggest questions facing mankind: are we alone in the universe?

As Nasa's James Webb Space Telescope (JWST) and other new giant telescopes come online, they will need fresh strategies to look for evidence of life on other planets.

A just-published, Nasa-funded study led by Auckland-raised doctoral student Joshua Krissansen-Totton has found one simple approach that might be more promising than just looking for oxygen.

The idea of looking for unusual combinations of atmospheric gases as a sign of life has a long history, spanning back to the 1960s and the early days of solar system exploration.

The concept was built on the logic that life produced waste gases that could alter the composition of a planet's atmosphere, therefore revealing the presence of life.

Until now, much focus has been on oxygen as a sign of life, or oxygen in surprising combinations with other gases.

"Looking for oxygen makes a lot of sense," said Krissansen-Totton, a University of Auckland graduate now based at the University of Washington in the US.

"Virtually all the oxygen in Earth's atmosphere was produced by photosynthesis, and it is extremely difficult to maintain oxygen-rich atmospheres without life."

But, he added, life-hunting scientists shouldn't place all of their eggs in one basket.

Even if life was common in the cosmos, oxygen rich atmospheres might be rare.

Oxygen-making photosynthesis was incredibly complex and only evolved once on Earth.

Further, oxygen could take a long time to accumulate in a planet's atmosphere.

"We know that Earth's atmosphere was virtually oxygen free for the first half of its history; it has only been for the last one eighth of Earth history that oxygen levels have been high enough to be easily detectable across interstellar distances," he said.

"This motivated us to try and find alternative combinations of gases that could reveal life on planets around other stars."

The new study looked at the history of life on Earth, the one inhabited planet we know of, to find times where the planet's atmosphere contained a mixture of gases that were out of equilibrium and could exist only in the presence of living organisms — anything from pond scum to giant trees.

By taking a longer view, Krissansen-Totton and his colleagues identified a new combination of gases that would provide evidence of life: methane plus carbon dioxide, minus carbon monoxide.

Future telescopes like the James Webb Space Telescope (right) will observe the atmospheres of distant planets to seek evidence of life. Earth (top left) has several gases in its atmosphere that reveal the presence of life, primarily oxygen and ozone. A new study finds that for the early Earth (bottom left), the combination of abundant methane and carbon dioxide would provide an alternative sign of life. Image / Nasa/Wikimedia Commons/Joshua Krissansen-Totton

The paper, published in the journal Science Advances, looked at all the ways that a planet could produce methane — from asteroid impacts, outgassing from the planet's interior, reactions of rocks and water — and found that it would be hard to produce a lot of methane on a rocky, Earth-like planet without any living organisms.

If methane and carbon dioxide were detected together, especially without carbon monoxide, that chemical imbalance could signal life.

The carbon atoms in the two molecules represented opposite levels of oxidation.

Carbon dioxide held as many oxygen molecules as it could, while the carbon in methane lacked oxygen and instead had oxygen's chemical adversary, hydrogen.

"So you've got these extreme levels of oxidation," Krissansen-Totton explained.

"And it's hard to do that through non-biological processes without also producing carbon monoxide, which is intermediate."

For example, planets with volcanoes that belched out carbon dioxide and methane would also likely belch out carbon monoxide.

What's more, carbon monoxide tended not to build up in the atmosphere of a planet that harboured life.

"Carbon monoxide is a gas that would be readily eaten by microbes," Krissansen-Totton said.

"So if carbon monoxide were abundant, that would be a clue that perhaps you're looking at a planet that doesn't have biology."

He and the other authors agreed that oxygen is a good way to look for signs of life - but thought this new combination was at least as likely to pop up through the new telescopes' sights.

"Life that makes methane uses a simple metabolism, is ubiquitous, and has been around through much of Earth's history," Krissansen-Totton said.

"It's an easy thing to do so it's potentially more common than oxygen-producing life.

"This is definitely something we should be looking for as new telescopes come online."

Next year, the JWST will launch and begin observing potentially habitable planets around nearby stars.

The instruments on the telescope were ideally suited for detecting atmospheric methane and carbon dioxide, and it could be possible the new biosignature combination would prove the first sign of life found beyond Earth.

"We'll need to get lucky though," said Krissansen-Totton, adding that the JWST would only look at a handful of potentially habitable planets.

"The observations would be extremely challenging to make.

"Therefore, we might have to wait for the large ground and space-based telescopes of the 2020s and 2030s to do a thorough search for nearby life."