Currently, the best way to search for alien life in other star systems is by looking for biosignatures: the presence of particular molecules in the atmospheres of distant worlds. Many substances are being considered for such a role – oxygen is a prime candidate, and methane too. Work on phosphine as a possible biosignature has led to a whole debacle about Venus and life in its clouds.
Now, researchers from the University of California, Riverside have put forward the case for another type of gas: nitrous oxide (N2O), commonly known as laughing gas. There are several biological processes that produce this substance, and models produced by the team suggest that it might be possible to detect it in the atmospheres of nearby exoplanets using JWST.
Fewer researchers have seriously considered nitrous oxide, but we think that may be a mistake.Eddie Schwieterman
"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," lead author Eddie Schwieterman, an astrobiologist in UCR's Department of Earth and Planetary Sciences, said in a statement.
The production of nitrous oxide is mostly due to microorganisms – some little beings can use nitrates to power their cellular metabolism, releasing laughing gas in the process.
"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."
This is where telescopes might spot it. Previous studies have discounted the possibility of seeing nitrous oxide because, on modern-day Earth, it is not very abundant. However, the team says this conclusion doesn’t take into account how exoplanets might have conditions more suitable for the creation of this gas. Also, stars dimmer than the Sun would less likely break this gas apart.
"This conclusion doesn't account for periods in Earth's history where ocean conditions would have allowed for much greater biological release of N2O. Conditions in those periods might mirror where an exoplanet is today," Schwieterman said.
The team is confident that nitrous oxide is a serious contender for biosignature to be investigated elsewhere in the galaxy.
"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," concluded Schwieterman.
The work was published today in The Astrophysical Journal.