Every now and then, scientists provide their own estimates on the probability of finding alien life on an exoplanet out in space. However, new estimates found by a new study suggest the probability of discovering alien life is less likely than was previously thought.
The new study led by a team at UC Riverside found that a buildup of toxic gases in the atmospheres of most planets, makes them unfit for rich and diverse life such as we have on Earth.
So far, the search for extraterrestrial life has focused on studying planets located in the so-called “habitable zone,” which includes a suitable distance from the host star where the planet’s atmosphere won’t evaporate, neither too far away for the planet to freeze. As well as liquid water, it would need some carbon-rich material which should be basic enough for supporting single-cell life.
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The team published their study in The Astrophysical Journal, describing that the predicted levels of certain toxic gases reduce the number of planets eligible for life as we know it, thus making the probability of alien life more unlikely.
“This is the first time the physiological limits of life on Earth have been considered to predict the distribution of complex life elsewhere in the universe,” Timothy Lyons, one of the study’s co-authors, a distinguished professor of biogeochemistry in UCR’s Department of Earth and Planetary Sciences, and director of the Alternative Earths Astrobiology Center, said in a statement.
The team used a computer model which analyzes atmospheric climate and photochemistry on different planets. The gas the team considered first was carbon dioxide. While carbon is essential for microbes to feed on, carbon dioxide in excess amounts can be deadly. However, planets that are too far from their host star would need much more carbon dioxide (which is a greenhouse gas) to prevent freezing temperatures on the planet.
“To sustain liquid water at the outer edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth has today,” said Edward Schwieterman, the study’s lead author and a NASA Postdoctoral Program fellow. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”
Researchers estimate that no safe zone for alien life exists among certain stars, including the Sun’s closest neighbors, Proxima Centauri and TRAPPIST-1, because the type and intensity of ultraviolet radiation these stars emit is colder, leading to high concentrations of carbon monoxide, an extremely dangerous gas, and which binds to hemoglobin in animal blood. That said, with the existence of toxic gases around the exoplanets we thought were most suitable for life, the probability of alien life is vastly reduced.
“I think showing how rare and special our planet is only enhances the case for protecting it,” Schwieterman said. “As far as we know, Earth is the only planet in the universe that can sustain human life.”