The current search for life on planets outside our own solar system consists of analyzing the chemical signatures in their atmospheres, transmitted via the light from the planet’s host star as it either shines through, or is reflected off of, those atmospheres. Researchers look for gases that could be produced by biological processes, such as methane or oxygen, some sort of sign that something is metabolizing on a planet far, far away. But these chemical signatures have a major drawback, in that many of these chemical compounds can also be generated by non-biological means; additionally, there is also the potential that truly alien organisms might produce (or consume) gases that we don’t associate with biological processes, meaning we might be looking at a sign of life, but not realize what it means.

Researchers at the University of California, Riverside’s Alternative Earths Astrobiology Center, have come up with a method that may help clarify potential false-positives and false-negatives such as these, by looking at seasonal changes in the chemical makeup of an exoplanet’s atmosphere. The gases present in Earth’s atmosphere change with the seasons, and quite dramatically so, as the vegetation in each hemisphere goes either dormant with the onset of winter, or resumes activity in the summer.

"Atmospheric seasonality is a promising biosignature because it is biologically modulated on Earth and is likely to occur on other inhabited worlds," explains Stephanie Olson, a UCR’s Department of Earth Sciences graduate student and lead study author.

"Inferring life based on seasonality wouldn’t require a detailed understanding of alien biochemistry because it arises as a biological response to seasonal changes in the environment, rather than as a consequence of a specific biological activity that might be unique to the Earth." Basically, looking for what life does, rather than for what we think life is, might broaden our chances for finding worlds that harbor life.

In addition to using present-day Earth as an example for these seasonal changes in atmospheric chemistry, they also provided models for Earth as it existed billions of years ago, when there was very little oxygen in the atmosphere, as an example for a potential proto-Earth. Interestingly, their computer modeling indicated that ozone might provide a better indication of seasonal variability than oxygen itself, especially on a planet where life is just beginning to take hold.

"It’s really important that we accurately model these kinds of scenarios now, so the space and ground-based telescopes of the future can be designed to identify the most promising biosignatures," said Edward Schwieterman, a NASA Postdoctoral Program fellow, also at UCR. "In the case of ozone, we would need telescopes to include ultraviolet capabilities to easily detect it."

For those interested in a visual representation of the seasonal change in CO2 concentrations in Earth’s own atmosphere, here are two interactive examples, courtesy of Summer in the Northern Hemisphere (August 2017), and late winter (April 2018).

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