Distant worlds teeming with simple life forms could be detected by the presence of a gas that’s deadly to humans, a new study suggests.
High levels of Carbon monoxide (CO) may be perfect for lifeforms on planets similar to Earth when organisms began to emerge around 3.5 billion years ago.
An atmosphere with high concentrations of CO would be deadly for complex forms of life, but it could signal alien microbes on other planets.
Nasa‘s James Webb Space Telescope, scheduled for launch in March 2021, will be able to examine CO concentrations in space.
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Distant worlds teeming with simple life forms could be detected by the presence of a gas that’s deadly to humans, a new study suggests (artist’s impression)
In humans, CO reduces the amount of oxygen that can be transported in the blood stream to critical organs like the heart and brain.
At very high levels, which are possible indoors or in other enclosed environments, CO can cause dizziness, confusion, unconsciousness and death.
The deadly gas is completely invisible, has no smell or taste and the only definitive way to detect a carbon monoxide leak is with a special alarm.
However, in the early days of the planet simple lifeforms were able to thrive in its presences.
Dr Edward Schwieterman, the study’s lead author and a NASA fellow at the University of California Riversides, said: ‘With the launch of the James Webb Space Telescope two years from now, astronomers will be able to analyse the atmospheres of some rocky exoplanets.
‘It would be a shame to overlook an inhabited world because we did not consider all the possibilities.’
Dr Schwieterman’s team used computer models of chemistry found in areas on Earth where live thrives, as well as the atmosphere.
They used this data to identify two scenarios in which carbon monoxide could readily build up in the atmospheres of living planets.
In the first scenario, the team found answers in our own planet’s past.
Earth’s current atmosphere prevents carbon monoxide from accumulating because the gas is quickly converted to carbon dioxide.
But three billion years ago, the world was a very different place.
The oceans were already teeming with microbial life and the atmosphere was nearly devoid of oxygen and the sun was much dimmer.
The James Webb Space Telescope (pictured) from Nasa is now set to fly in 2021 and will characterise planets for levels of carbon dioxide and potential signs of alien lifeforms
The team’s models reveal that this ancient version of inhabited Earth could maintain carbon monoxide levels of roughly 100 parts per million (ppm) – millions of times higher than current levels.
Professor Timothy Lyons, one of the study’s co-authors, said: ‘That means we could expect high carbon monoxide abundances in the atmospheres of inhabited but oxygen-poor exoplanets orbiting stars like our own sun.
‘This is a perfect example of our team’s mission to use the Earth’s past as a guide in the search for life elsewhere in the universe.’
A second scenario is even more favourable for the buildup of carbon monoxide: the photochemistry around red dwarf stars like Proxima Centauri, the star nearest our sun at 4.2 light years away.
The team’s models predict that if a planet around such a star were inhabited and rich in oxygen, then we should expect the abundance of carbon monoxide to be extremely high – anywhere from hundreds of ppm to several percent.
Dr Schwieterman added, ‘Given the different astrophysical context for these planets, we should not be surprised to find microbial biospheres promoting high levels of carbon monoxide’.
‘However, these would certainly not be good places for human or animal life as we know it on Earth.’
Earth-sized, rocky planets have been discovered orbiting in the habitable zone of Proxima Centauri and other similar stars, meaning they could harbour liquid water, an essential ingredient for life.
Such planets are likely targets for further characterisation by the James Webb Space Telescope, scheduled for launch in March 2021.
The current study is one component of a broad effort to prepare for these future missions by cataloguing different combinations of atmospheric gases that might be evidence of an inhabited world – so-called biosignature gases.
Some gases, such as carbon monoxide, had been proposed previously as ‘antibiosignatures’— evidence that a planet is not inhabited – if remotely detectable at sufficient abundance. But those assumptions only apply in specific cases.
Dr Schwieterman added: ‘Although other studies have done exoplanet photochemical modeling that includes carbon monoxide, no one had focused on carbon monoxide on Earth-like exoplanets in such a systematic way,’
‘Now we have a guidebook for determining what levels of carbon monoxide are compatible with a photosynthetic biosphere.’
The full study is published in The Astrophysical Journal.
