'Safe Zone' Around Stars Where Alien Life Could Exist May Be Much Smaller Than We Thought

To date, scientists have identified nearly 4,000 exoplanets, a handful of which could potentially contain the right conditions for life to exist.

These planets lie in the so-called habitable zone (HZ) which is usually defined as the region around a host star within which liquid water—a key component for life as we know it—could exist on the surface.

But according to a study published in The Astrophysical Journal, the number of planets in the habitable zone which may have the potential to host complex life may be lower than once thought.

"The fundamental questions we were seeking to answer were: 'Who is the habitable zone habitable for' and 'Where in the habitable zone could complex life —such as animals and humans—reside?'" Edward Schwieterman, lead author of the study from the University of California, Riverside, told Newsweek. "The answers to these questions inform the potential distribution of complex life in the universe and the potential for planetary environments to be conducive to the evolution of intelligent life."

"In past studies, the key focus has been to determine the range of conditions amenable for simple, microbial life," he said. "This includes the requirement for above freezing temperatures, which allow the possibility of surface liquid water, a key ingredient for life."

For their study, the researchers used computer models to simulate various conditions on a number of planets.

"We used computational models for planetary climate and atmospheric chemistry," Schwieterman said. "These same models have been used by others in the field for a variety of purposes, such as for calculating the traditional habitable zone boundaries and predicting the abundance of biosignature gases in exoplanetary atmospheres."

These models showed that the atmospheres of many planets in the habitable zone could contain high levels of toxic gases that would limit the "safe zone" in which complex life could exist.

"Imagine a 'habitable zone for complex life' defined as a safe zone where it would be plausible to support rich ecosystems like we find on Earth today," Timothy, Lyons, a co-author of the study from Riverside, said in a statement. "Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined."

Notably, the team found that substantially more carbon dioxide—a greenhouse gas—than is present in Earth's atmosphere today is required to maintain clement temperatures for most planets in the habitable zone.

"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," Schwieterman said in the statement. "That's far beyond the levels known to be toxic to human and animal life on Earth."

On Earth, most complex aerobic life—organisms that survive and grow in oxygen-filled environments—are limited by even relatively low levels of carbon dioxide. Using this knowledge, the scientists predicted that just by taking into account carbon dioxide toxicity, only up to half of the traditional habitable zone would actually be capable of hosting simple life. And when it comes to more complex life, the "safe zone" is reduced even further to around a third of the traditional habitable zone.

"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," Lyons said.

Furthermore, the team found that certain cooler, dimmer stars—such as Proxima Centauri and TRAPPIST-1—emit a certain kind of ultraviolet radiation which means there is no safe zone at all within their habitable zone. This radiation could enable high build-ups of carbon monoxide—another toxic gas—in the atmospheres of planets in the habitable zone, making it difficult for life as we know it to evolve. On Earth, our hotter, brighter sun destroys carbon monoxide in the atmosphere.

The researchers say that their findings could have significant implications in the search for life on other planets.

"Our discoveries provide one way to decide which of these myriad planets we should observe in more detail," Christopher Reinhard, another co-author of the study, said in the statement. "We could identify otherwise habitable planets with carbon dioxide or carbon monoxide levels that are likely too high to support complex life."

This article was updated to include additional comments from Edward Schwieterman.

Kepler-186f, exoplanet
This artist's concept depicts Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone—a range of distance from a star where liquid water might pool on the planet's surface. NASA