The huge bloom in the exoplanet's discoveries over the last decade gives us more and more hope that we will soon find life in another world. While water remains the most important sign that extraterrestrial life is feasible, scientists are looking for many other chemical elements and compounds that could enhance the ability of extraterrestrial life to evolve and maintain elsewhere. Oxygen is obviously such a "biological signature," given how important it is to fill life here on this planet.
But maybe we put too much stock in the air we breathe. A new study published in ACS Earth and Space Chemistry suggests that the presence of atmospheric oxygen on another planet is far from a certain sign.
"The presence of oxygen in the Earth's atmosphere, in significant quantities, is due to the presence of life," says Nikole Lewis, an exoplanet scientist at Cornell University and a colleague of the new study. While life could certainly be without significant access to oxygen, "most of the current thinking about how life is traced to other planets is centered on finding planets with atmospheres very similar to Earth's," he says.
However, before we know whether a gas or a combination of gases suggests life, "we must fully understand the chemistry that is going on on a planet," says Chao He, a researcher at Johns Hopkins University and lead author of the new publication. "Our study provides some evidence for atmospheric chemistry" and suggests that some processes could easily produce oxygen without biology. Scientists may need to think that oxygen could be a false flag for signs of a foreign life.
Of course, this is not easy to investigate. He and his colleagues exploited the special Planetary HAZE Research or PHAZER: an experimental chamber that can simulate a wide range of different atmospheric chemistry conditions, from the cold surface of Pluto to the incredibly warm high heights of Venus. The idea is to accurately reveal mixed gases to a source of high energy (plasma or ultraviolet radiation, both emitted by a star) to see if flares (small particles produced by photochemical reactions) are formed and if it changes its atmospheric chemistry.
"The idea is that we can not just simulate the chemical processes that occur in these atmospheres, but also differentiate them in a systematic way so that we can understand what processes really dominate the things we really can see with spacecraft and telescopes" Hörst , a professor of planetary science from Johns Hopkins University and co-colleague of the new work.
He and his team ended up testing nine different gas mixtures simulating predicted atmospheric chemical processes on Super-Earth and mini-Neptune outposts (the most abundant types of exoplanets in the Milky Way galaxy). All of these atmospheres have different amounts of carbon dioxide, hydrogen and water and ranged between 80 and 700 degrees Fahrenheit.
Finally, the team found that exposure to UV radiation and plasma altered selected gas mixtures in such a way as to produce various types of biological labeling – including oxygen and other organic compounds – clearly and abiotically. In other words, they showed that living creatures are not essential for the production of free oxygen – it could be the result of photochemistry, suggesting that oxygen may not be an inherently strong sign of life.
"It will be important for future remarks that assertions of life are able to exclude that these types of biological signature are not generated through abiotic sources," Lewis says. "We should continue to explore how it is possible to produce biologically false positives in the atmosphere of the exoplanets, but also to determine what combinational observations cover what wavelengths will allow us to exclude false positives of such."
The study is hampered by the fact that chemical simulations ran only for a few days and that gas mixtures were based on predictions of what exoplanets could have rather than in guiding observations (which we do not have the technology to do today). Some of these limitations will be resolved over the next decade when instruments such as the James Webb space telescope can measure the chemical compositions of some distant worlds but these findings are a good reminder that we should not necessarily restrict ourselves to exoplanets that fit a Earth – like the pattern.
The biggest impact of the study may simply show how unlikely we are to be able to use a simple measure to find alien life. "The experiments themselves are not the final answer," warns Hörst, "but it is an important part of a puzzle that includes observations, computer models and laboratory experiments." We are not the first or only the people who say this but will be really, really, Really causing the search for life by measuring the atmospheric composition only ". It emphasizes the need to explore deeper understanding of how other factors such as volcanic activity, comet conflicts and the new chemistry we need to consider can contribute to the production of biological signals, the absence of biology.
If the truth is out there, it's more complicated than we ever predicted.