2.4 billion years ago, a phenomenon known as the Great Oxidation Event breathed a little-known gas by the name of oxygen into Earth’s atmosphere, birthing life into the world. While we Earthlings wouldn’t last very long without it, it doesn’t necessarily mean that oxygen is the universal gas needed to signal life elsewhere.

Now researchers have identified a new gaseous recipe that could help scientists to better search for life on exoplanets outside our solar system.

Publishing their findings in Science Advances, the researchers from the University of Washington looked back through Earth’s long history and searched for times when our planet’s atmosphere contained a mixture of gases that were out of whack, but some form of life existed.

“If you were an alien looking at Earth from a distant star, it would be very obvious that the modern Earth hosts life due to large amounts of atmospheric oxygen,” Joshua Krissansen-Totton, corresponding author of the study and doctoral student of Earth and Space Sciences at the University of Washington, explained to Alphr. “To investigate this, we took best estimates of what the composition of Earth’s atmosphere has been like at various times in its history and calculated whether that atmosphere was out of chemical equilibrium. An atmosphere is said to be in chemical equilibrium if its gases are all completely reacted to reach a steady state where no further chemical reactions are possible without inputting energy.”

They found that the biggest contributor to Earth’s early disequilibrium was a concoction of gases which included methane, carbon dioxide, nitrogen and liquid water, with an absence of carbon monoxide. These gases aren’t able to co-exist for long stretches of time, as the methane should always get destroyed. However, in Earth’s early history, the methane gas kept on being replenished by life.

The researchers found that it was extremely challenging to make large amounts of methane on a rocky planet without life being there to replenish it. If methane and carbon dioxide are found together, then there’s a chemical imbalance which signals life. Another indicator is the telling absence of carbon monoxide, which they find tends not to build up in the atmosphere of a planet which is home to life. Find this exact recipe, and we may find life on other planets.

READ NEXT: In an infinite universe, where are all the aliens?

In the first half of Earth’s timeline, despite the presence of life, there was virtually no oxygen in our planet’s atmosphere. Even then, today’s levels of oxygen has only been observed in the last eighth of our planet’s history, so it would be presumptive to suggest that it’s purely oxygen that gives any indication of life.

“For that reason alone, it makes sense to consider alternative biosignatures and not put all our eggs in one basket,” Krissansen-Totton adds. “The methane-carbon dioxide combination we propose is potentially more common than oxygen. This is because oxygen-producing photosynthesis is a very complex metabolism that took a long time to emerge on Earth and only evolved once. In contrast, methane-production is a relatively simple metabolism, and there is evidence suggesting it evolved almost immediately after the origin of life. All that is needed to make methane is carbon dioxide and hydrogen gases, which we expect to be emitted by volcanoes on terrestrial planets.”

And we could be closer than ever before to finding life forms outside of Earth. NASA is about to launch the James Webb Space Telescope (JWST) next year, and time will be spent observing Earth-sized and potentially life-habitable planets around nearby stars.

“JWST (and other future large telescopes) will measure the composition of these exoplanet atmospheres using remote spectroscopy,” Krissansen-Totton told Alphr. “Different gases absorb light at specific diagnostic wavelengths, and therefore, by looking at the spectrum of light from a planet, astronomers can determine what gases are present and in what quantity.”

Once the gases have been determined, the researchers’ biosignature combination can be used to interpret the results, and decide on whether those gases are due to the presence of life or not.

“It will be the job of planetary scientists and astrobiologists to interpret those observations,” he continued.

The James Webb telescope is the successor to the Hubble telescope and is now the largest space telescope in the world. It was supposed to launch in October this year, but NASA was forced to delay it until February to March 2019, due to the need for further testing.

Come 2019 though, and the JWST will turn its gaze to our solar system’s two most promising candidates for life. These candidates are the two icy moons: Jupiter’s Europa and Saturn’s Enceladus. Both the moons contain oceans of water, which is one of the researchers’ core ingredients for life. Europa and Enceladus have previously shown evidence of liquid geyser-like plumes that shoot out through their thick icy surfaces, leading researchers to think that they could provide sources of nutrients and heat to any lifeforms living there. Until next year though, we’ll have to sit with bated breaths here on our Oxygen-filled planet to see if the Webb telescope picks up signs of the atmospheric disequilibrium that mirror Earth’s early history.