Laughing gas could be a key sign of life on alien planets – but it’s not because aliens are having their teeth extracted under anaesthetic.
Instead, the gas is commonly generated by lifeforms on Earth: microorganisms are constantly transforming other nitrogen compounds into nitrous oxide, or N2O, a metabolic process that can yield useful cellular energy.
Under the light of other stars, the compound might be more liable to build up in the atmosphere and detectable by telescopes.
So scientists now argue that – just like other gases such as oxygen or methane – nitrous oxide should be considered as a ‘biosignature’, a chemical compound in a planet’s atmosphere which can indicate life.
Biosignatures typically include gases found in abundance in Earth's atmosphere today.
Eddie Schwieterman, an astrobiologist at University of California – Riverside's Department of Earth and Planetary Sciences, said: "There's been a lot of thought put into oxygen and methane as biosignatures.
“Fewer researchers have seriously considered nitrous oxide, but we think that may be a mistake.”
The researchers determined how much nitrous oxide living things on a planet similar to Earth could possibly produce.
They then made models simulating that planet around different kinds of stars and determined amounts of N2O that could be detected by an observatory like the James Webb Space Telescope.
Schwieterman said: "In a star system like TRAPPIST-1, the nearest and best system to observe the atmospheres of rocky planets, you could potentially detect nitrous oxide at levels comparable to CO2 or methane,".
"Life generates nitrogen waste products that are converted by some microorganisms into nitrates. In a fish tank, these nitrates build up, which is why you have to change the water," Schwieterman added.
"However, under the right conditions in the ocean, certain bacteria can convert those nitrates into N2O. The gas then leaks into the atmosphere."
Seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth.
The researchers admit that N2O could be detected in an atmosphere and still not indicate life – but there’s a way around this.
A small amount of nitrous oxide is created by lightning, for example. But alongside N2O, lightning also creates nitrogen dioxide, which would offer astrobiologists a clue that non-living weather or geological processes created the gas.
Schwieterman added that common stars like K and M dwarfs produce a light spectrum that is less effective at breaking up the N2O molecule than our sun is.
These two effects combined could greatly increase the predicted amount of this biosignature gas on an inhabited world.
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