Nitrous Oxide, aka “Laughing gas”, Could be an Indication of Life in an Exoplanet

A team of astronomers have proposed to hunt for signs of life by looking for the signature of nitrous oxide in alien atmospheres. It’s laughing gas, but it’s no joke.

Many astronomers worldwide are on the hunt for signs of life outside of the Earth. One way to do this is to search for so-called biosignatures, which are chemical compounds or elements that are produced in abundance by living creatures. A world uninhabited by life would have an equilibrium atmosphere that would not include these kinds of elements.

Researchers have traditionally focused on oxygen and carbon dioxide, as these are natural byproducts of life on Earth, and life on Earth appears pretty successful, so there’s plenty of those two in our atmosphere. 

But new research suggests that focusing on these two elements limits our scope. The researchers behind the study argue that there were periods in the Earth’s own history where different chemicals would have been more common in our atmosphere than they are today, and so they may be viable biosignature candidates. 

“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,” said Eddie Schwieterman, an astrobiologist in UCR’s Department of Earth and Planetary Sciences. 

The researchers pointed to nitrous oxide, N2O, commonly known as laughing gas. Many biological reactions generate nitrogen as a byproduct which can accumulate in ocean water. In the right conditions, certain bacteria can transform these nitrates into N2O, which then dissolves out of the oceans and builds up in the atmosphere.

Our Sun is very good at breaking up nitrous oxide, and so there’s not a lot of laughing gas in our atmosphere. But different planets in different stages of their evolution, or ones orbiting different kinds of stars that are less efficient at breaking it up, may have an abundance of that molecule.

The astronomers found that the James Webb Space Telescope could potentially detect nitrous oxide in nearby exoplanets. “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,” Schwieterman said. 

But just finding it wouldn’t be a sure bet that there’s life on an exoplanet. There are natural sources of N2O, like lightning strikes. But those natural processes also produce a lot of nitrogen dioxide, which would be a clue that life would was not responsible for that in an alien atmosphere.

If we find laughing gas alone and in abundance in an alien atmosphere, it may indeed be something serious.