We might have just found an entirely new way of spotting aliens.
Scientists have proposed a new way of looking out for marks of aliens in the universe. And it could help us see life forms we'd completely miss otherwise.
Space agencies including Nasa have been active in launching new tools to study the universe, such as the James Webb Telescope. That will provide information on the atmospheric makeup of planets far away – but we might not be sure how to use that information.
Until now, scientists have mostly been looking for oxygen in the atmosphere. If that's found, then it's likely that there's the chance for life on that planet, since we know from life on Earth that oxygen is key.
But we might be missing other important markers – known as biosignatures – that could indicate such worlds are supporting life. As such, planets might have life on them that we wouldn't spot using just oxygen.
"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 Joshua Krissansen-Totton, an author on the paper published in Science Advances. "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."
To do the research, the scientists looked at the history of life on Earth, and the kinds of gases that were around when it was. They found that the planet had a complex mix of different gases, not only oxygen, and that looking for that cocktail could be a far more reliable marker of life on a planet.
"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 co-author David Catling, a UW professor of Earth and space sciences. "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 scientists explored how a planet could have an imbalance of methane, including possible events like asteroid impacts or rocks interacting with water. But they found it would be very difficult to produce a lot of the gas on a rockey, Earth-like planet – unless it had living organisms on it.
The processes that generate both methane and carbon dioxide, such as big volcanic eruptions, would also tend to produce carbon monoxide alongside them. But carbon monoxide would be eaten up by microbes living on those planets.
"So if carbon monoxide were abundant, that would be a clue that perhaps you're looking at a planet that doesn't have biology," said Mr Krissansen-Totton.
That means that if we were able to find methane and carbon dioxide on planets that didn't have much carbon monoxide, it could be a good way of spotting aliens even at some distance away, they said.
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