Last updated February 21, 2018 at 11:03 am
The search for oxygen may have been a red herring – after all many life forms have survived without it, even on Earth.
Until now, one strategy scientists have used to look for life on other planets is trying to detect oxygen in their atmospheres. But a new paper has suggested a new recipe for providing evidence that a distant planet harbors life.
It involves looking not for oxygen, but for the presence of two or more long-term incompatible gases, such as methane and carbon dioxide – a phenomenon known as atmospheric chemical disequilibrium.
Looking to Earth for inspiration
Whether oxygen is present or not in a planet’s atmosphere is not enough to determine if life is present.
Earth’s atmosphere was virtually oxygen-free in the Archean period (4 to 2.5 billion years ago), despite there being many forms of life.
“This idea of looking for atmospheric oxygen as a biosignature has been around for a long time. And it’s a good strategy – it’s very hard to make much oxygen without life,” said one of the authors, Joshua Krissansen-Totton from the University of Washington. “But we don’t want to put all our eggs in one basket. Even if life is common in the cosmos, we have no idea if it will be life that makes oxygen. The biochemistry of oxygen production is very complex and could be quite rare.”
By studying the history of life on Earth, the one inhabited planet we know of, the researchers instead found times where the planet’s atmosphere contained a mixture of gases that were out of balance. The shift could exist only in the presence of living organisms — anything from pond scum to giant redwoods.
By studying these shifts in balance, the researchers hit upon a new combination of gases that would provide evidence of life: methane plus carbon dioxide, but without carbon monoxide.
The chemistry of life
The scientists examined the ways that a planet could produce methane – including asteroid impacts, expulsion from the interior of the planet, and chemical reactions between rocks and water – and found that it would be difficult to explain a lot of methane on a rocky, Earth-like planet without a contribution from living organisms.
“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.”
If methane and carbon dioxide are detected together, especially without carbon monoxide, that would be a chemical imbalance that signals life. The carbon atoms in the two molecules represent opposite ends of a chemical reaction called oxidation. Carbon dioxide (CO2) holds as many oxygen molecules as it can, while the carbon in methane (CH4) contains no oxygen and instead has hydrogen. To convert from one chemical to the other involves the creation of carbon monoxide (CO).
However, carbon monoxide tends not to build up in the atmosphere of a planet that harbors 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.”
The scientist’s new formula for life still requires the presence of water, which is currently used as an indicator of conditions that would support life. They also still agree that oxygen is a valuable chemical to search for, but think that this new combination is at least as likely to be detected.
The search for life continues
Telescopes around the world search for life on other planets by detecting gases in the atmosphere of exoplanets, and this new scheme could increase the chances of finding a significant result. Additionally, the next generation of telescopes are about to come online – such as the James Webb Space Telescope, which are even more sensitive than ever before, so it’s important to use them in a way that will maximise our chances of finding life on an exoplanet, including examining exoplanets on the nearby Trappist system.
“We need to look for fairly abundant methane and carbon dioxide on a world that has liquid water at its surface, and find an absence of carbon monoxide,” said Professor David Catling, a co-author on the paper. “Our study shows that this combination would be a compelling sign of life. What’s exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future.”
The research has been published in Science Advances