Oxygen might not lead us to strangers after all

The huge proliferation of exoplanet discoveries over the last decade gives us more and more hope that we will soon find life on another world. While water remains the most important sign that an extraterrestrial life is possible, scientists are looking for many other elements and chemical compounds that could enhance the ability of extraterrestrial life to evolve and stay elsewhere. Oxygen is obviously one of those "biosignatures", given the importance of the complexity of life on this planet.

But maybe we put too much broth in the air we breathe. A new study published in ACS Chemistry Earth and Space suggests that the presence of atmospheric oxygen on another planet is far from being a sure sign.

"The presence of oxygen in the earth's atmosphere, in large quantities, is due to the presence of life," said Nikole Lewis, Cornell University's exoplanet scientist and co-author of the new study. Although life can certainly exist without significant access to oxygen, "most of the current thinking about the detection of life on other planets is focused on the search for planets whose atmosphere is very similar to that of the Earth, "she says.

However, before knowing whether a gas or a combination of gases indicates life, "we need to fully understand the chemistry that is happening on a planet," says Chao He, a researcher at Johns Hopkins University and lead author of the new article. "Our study provides insight into atmospheric chemistry" and suggests that some processes could easily produce oxygen without biology. Scientists may need to consider that oxygen could be a false flag for signs of extraterrestrial life.

Of course, this is not an easy question to consider. He and his colleagues have benefited from Johns Hopkins' global research HAZE, or PHAZER: an experimental chamber capable of simulating a wide range of atmospheric chemistry conditions, from the glacial surface of Pluto to the incredibly warm high altitudes of Venus. The idea is to accurately expose mixed gases to a high energy source (plasma or ultraviolet that can be emitted by a star) to see if fogs (small particles produced by photochemical reactions) will form, and if that will change the chemistry of the atmosphere itself.

"The idea is to be able not only to simulate the chemical processes taking place in these atmospheres, but also to vary them in a systematic way to try to understand which processes dominate what we can actually observe with spacecraft and telescopes," explains Sarah Hörst, professor of planetary sciences at Johns Hopkins University and co-author of the new article.

He and his team ended up testing nine different gas mixtures simulating predicted atmospheric chemistry on super-terrestrial and mini-Neptune exoplanets (the most abundant types of exoplanets in the Milky Way galaxy). These atmospheres all had different amounts of carbon dioxide, hydrogen, and water, and ranged from 80 to 700 degrees Fahrenheit.

In the end, the team found that UV and plasma exposure altered the selected gas mixtures so that they produced different types of biosignatures, including oxygen and phosphorus. other organic compounds, distinctly and abiotically. In other words, they showed that living creatures are not needed to produce free oxygen. This may be the result of photochemistry, suggesting that oxygen is not in itself a sign of robust life.

"It will be important for future observations that detections of life be able to eliminate the fact that these biosignature species are not created by abiotic sources," says Lewis. "We should continue to look for ways to produce false biological positives in exoplanet atmospheres, but also to determine what combinations of observations covering what wavelengths would allow us to exclude such false positives."

This study is hampered by the fact that the chemical simulations lasted only a few days and that the gas mixtures were based on predictions of what the exoplanets might have, rather than on direct observations (for the moment we we do not have the necessary technology). . Some of these limitations will be solved in the next decade, when instruments such as the James Webb Space Telescope will be able to measure the chemical compositions of some distant worlds, but these results remind us that we should not necessarily limit ourselves to the exoplanets that correspond to a Land. -like model.

The main implications of the study may be simply to show how unlikely it is for us to use a single measure to find extraterrestrial life. "The experiments themselves are not the final solution," warns Hörst, "but they are an important piece of a puzzle including observations, computer models and laboratory experiments. We are not the first or the only one to say that, but it will be really, really. really difficult to search for life by measuring only atmospheric composition. She stresses the need to better understand how other factors, such as volcanic activity, comet collisions and new chemistry that we have not yet considered, can contribute to the production of biosignature, even the 39, absence of biology.

If the truth is revealed, it is more complicated than we had ever predicted.