To search for signs of alien life, we may have to look into Earth鈥檚 past. We don鈥檛 have the technology to directly detect plants or animals, much less microbes, on alien worlds, so we have to look for the ways that they might affect their planet鈥檚 atmosphere. But what should we be looking for? New simulations of the gases in ancient Earth鈥檚 atmosphere can give us a hint.
Searching for oxygen is a good way to look for life, as there are few ways to produce large amounts of it without photosynthesis. But we know there can still be photosynthesis and life without detectable amounts of oxygen.
Geological studies show us that Earth likely did not have detectable oxygen levels between the Archaean聽period 鈥� from 4 to 2.5 billion years ago 鈥� and the Proterozoic period, which ended 500 million years ago. But the planet did host microbial life during that time.
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鈥淥xygen-rich atmospheres could be quite rare,鈥� says at the University of Washington in Seattle. 鈥淓ven if you were to take the Earth at a random place in its history, it might not have much oxygen. That鈥檚 why we don鈥檛 want to put all our eggs in one basket.鈥�
Follow the methane
When an exoplanet does not have oxygen to indicate life, some astrobiologists have suggested that we could look for a combination of gases that cannot exist together for more than about a decade unless something alive is constantly replenishing them.
To sort out which combinations of gas we ought to look for, Krissansen-Totton and his colleagues calculated which gases made up early Earth鈥檚 atmosphere and how they could coexist.
They found that a combination of methane, nitrogen, water, and carbon dioxide, could have been an early sign of life. 鈥淭hese four chemicals should not coexist 鈥� they should react and remove all the methane,鈥� says Krissansen-Totton. 鈥淥n the early Earth, the vast majority of methane was coming from microbes, and that was why it wasn鈥檛 disappearing.鈥�
The mechanism through which organisms produce methane is also much simpler than the one by which they make oxygen, which means that methane-producing life might be more common 鈥� thus easier to spot 鈥� than oxygen-producing life.
A new era of alien hunting
When it launches in 2019, the James Webb Space Telescope 鈥� along with several ground-based telescopes 鈥� will be able to examine the compositions聽 of exoplanet atmospheres. Krissansen-Totton says that they should not only hunt for the mixture of methane, nitrogen, water, and carbon dioxide, but also for a lack of carbon monoxide. Most non-biological processes that create both methane and carbon dioxide cannot also produce carbon monoxide.
Even if life on other worlds is聽very different from life here on Earth, Krissansen-Totton says it may still produce methane that can serve as a beacon. 鈥淟ife everywhere has to obey the same rules of physics and chemistry 鈥� it has to make a living in its environment,鈥� he says. 鈥淚t has to use the energy that鈥檚 available, so producing methane isn鈥檛 such an unlikely thing.鈥�
Science Advances
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