In a study published Wednesday in the Astrophysical Journal, scientists at the University of California suggested that something is missing from the typical list of chemicals that astrobiologists use to search for life on planets around other stars, and it is laughing gas.
Chemical compounds in the planet’s atmosphere that could indicate the presence of life are called “biosignatures”, and usually include gases that are abundant in Earth’s atmosphere today.
“There has been a lot of thinking about oxygen and methane as biological classifications,” says Eddie Schwitterman, an astrobiologist in the University of California’s Department of Earth and Planetary Sciences. “.
To reach this conclusion, Schwittermann led a team of researchers to determine the amount of nitrous oxide that organisms can produce on a planet similar to Earth, then make models that simulate that planet around different types of stars and determine the amounts of nitrous oxide that can be detected by an observatory such as the James Webb Space Telescope.
“In a star system like TRAPPIST-1, the closest and best system for observing the atmosphere of rocky planets, you can detect nitrous oxide at levels similar to carbon dioxide or methane,” Schwettermann says.
There are several ways in which organisms can form nitrous oxide, or N2O, as microorganisms are constantly converting other nitrogen compounds into N2O, a metabolic process that can produce useful cellular energy.
“Life generates nitrogen waste that is converted by some microorganisms into nitrate, and in a fish tank, this nitrate accumulates, which is why you have to change the water,” Schwittermann says.
“However, under the right conditions in the ocean, certain bacteria can convert that nitrate into nitrous oxide, and the gas then escapes into the atmosphere,” Schwittermann explains.
Under certain conditions, nitrous oxide can be detected in the atmosphere and still not indicate life, and Schwittermann’s team took this into account in their modeling, where a small amount of nitrous oxide is formed by lightning, for example.
But besides nitrous oxide, lightning also produces nitrogen dioxide, which would provide astrobiologists with evidence that nonliving weather or geological processes are generating the gas.
Others who have considered nitrous oxide as a gas with a biosignature that would be difficult to detect from afar, often based on the fact that nitrous oxide concentrations in Earth’s atmosphere today are not significant, and thus would also be difficult to detect elsewhere.
“This conclusion does not take into account periods in Earth’s history when ocean conditions allowed for much greater biological release of nitrous oxide,” Schwittermann says. “Conditions at those times may reflect where an exoplanet is today.”