Astronomers have discovered nearly 4,000 exoplanets to date and at least 50 of these alien worlds fall within the habitable zone of their host star. This means these planets are not too hot and not too cold to support liquid water on their surface, which as far as we know is necessary for all organic life. Actually discovering whether non-intelligent life exists on an exoplanet is a bit trickier, however.
Most exoplanet hunting telescopes are only able to tell basic things about a planet like its mass or distance from its host star, but a new generation of exoplanet telescopes like PLATO promise to reveal their subjects detail, including the composition of their atmospheres. The question, then, is what sorts of atmospheric markers would indicate the presence of extraterrestrial life?
In a new paper published Wednesday in Astrophysical Journal Letters, University of California-Riverside planetary scientist Stephanie Olson outlined a dynamic framework for detecting life based on how the composition of exoplanet atmospheres change during the seasons.
The model is based on Earth’s own atmospheric seasons, which change as the Earth rotates on its axis. For example, during the summer the composition of the northern hemisphere’s atmosphere shows a marked rise in oxygen and a corresponding decrease in carbon dioxide because of all the plant growth. Olson argues that similar signatures could reveal the presence of life on other planets.
“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,” Olson said in a statement.
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In the paper, Olson and her colleagues outlined how spectroscopy, which measures how chemicals interact with light, can be used to detect seasonal fluctuation of atmospheric oxygen, carbon dioxide, and methane levels on exoplanets. They also created a model of a planet with a low-oxygen atmosphere, similar to the Earth a few billion years ago. In this case, ozone—which is formed by reactions with oxygen in the atmosphere—would be a better marker of season variations of oxygen in the atmosphere than oxygen itself.
“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 at UC Riverside and a co-author of the paper. “In the case of ozone, we would need telescopes to include ultraviolet capabilities to easily detect it.”
Still, the system wouldn’t be perfect. Although atmospheric methane and oxygen are both promising signs of life, they aren’t a guarantee that life exists. In order to eliminate false positives, Olson suggested monitoring the fluctuations of these chemicals in the atmosphere over time, which would give a better indication of whether they were the result of life.
In the meantime, however, its up to the new generation of exoplanet telescopes coming online, like the recently launched TESS and upcoming PLATO telescopes, to find new exoplanets that may be harboring life.
With hundreds of new exoplanets being discovered every year, the odds of finding and recognizing signs of life elsewhere in the universe seem better than ever.