Scientists may need to rethink their estimates of how many planets outside our solar system could harbor a rich diversity of life.
In a new study, a team led by UC Riverside discovered that an accumulation of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it.
Traditionally, much of the search for extraterrestrial life has focused on what scientists call "habitable zone," defined as the range of distances from a hot star enough for liquid water to exist on a planet's surface. This description works for single-celled basic microbes – but not for complex creatures like animals, ranging from simple sponges to humans.
The team's work, published today in The Astrophysical Journal, shows that accounting for predicted levels of certain toxic gases restricts the safe zone to complex life by at least half – and in some cases, completely eliminates it.
"This is the first time that the physiological limits of life on Earth have been considered to predict the distribution of complex life in other parts of the universe," said Timothy Lyons, a co-author of the study, an illustrious professor of biogeochemistry at the Department of UCR. Earth and Planetary Sciences, and director of the Alternative Astrobiology Center for Earth, who sponsored the project.
"Imagine a habitable zone for complex life" defined as a safe zone, where it would be plausible to support rich ecosystems, as we find on Earth today, "explained Lyons. "Our results indicate that complex ecosystems like ours can not exist in most regions of the habitable zone as traditionally defined."
Using computational models to study the atmospheric and photochemical climate on a variety of planets, the team first considered carbon dioxide. Any diver knows that much of this gas in the body can be fatal. But planets far removed from their host star require carbon dioxide – a potent greenhouse gas – to keep temperatures above freezing. Earth included.
"To sustain liquid water on the periphery of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than Earth today," said Edward Schwieterman, lead author of the study and colleague of the Postdoctoral Program NASA. . "This is far beyond the levels known to be toxic to human and animal life on Earth."
The new study concludes that carbon dioxide toxicity alone restricts simple animal life to no more than half of the traditional habitable zone. For humans and other higher order animals, which are more sensitive, the safe zone shrinks to less than a third of that area.
In addition, no safe zone exists for certain stars, including two of the Sun's closest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation that these colder stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide binds to hemoglobin in animal blood – the compound that carries oxygen through the body. Even small amounts can cause the death of body cells due to lack of oxygen.
Carbon monoxide can not accumulate on Earth because our warmer, brighter sun generates chemical reactions in the atmosphere that destroy it quickly. While the team has recently concluded that microbial biospheres may be able to thrive on a planet with abundant carbon monoxide, Schwieterman emphasized that "these would certainly not be good places for human or animal life as we know it on Earth."
Scientists have confirmed about 4,000 planets orbiting stars other than the sun, but none of them will be able to visit them in person. They are simply too far away. The closest is Proxima Centauri b, which would take 54,400 years for the current spacecraft to arrive. Using telescopes to detect the abundance of certain gases in their atmospheres is one of the only ways to study these so-called exoplanets.
"Our findings provide a way to decide which of these myriads of planets we should look at in more detail," said Christopher Reinhard, a UCR graduate student and an assistant professor at the Georgia Institute of Technology, who co-authored the study. Team leader Alternative Earths. "We could identify habitable planets with levels of carbon dioxide or carbon monoxide that are probably too high to support complex life."
Findings from the team's previous work are already informing next-generation space missions, such as the Habitable Exoplanets Observatory proposed by NASA. For example, because oxygen is essential for complex life on Earth and can be detected remotely, the team has been studying how common it can be in different atmospheres of planets.
Beyond Earth, no planet in our solar system hosts life that can be characterized at a distance. If life exists in other parts of the solar system, Schwieterman explained, it is well below a rocky or icy surface. Thus, exoplanets may be our best hope for finding habitable worlds more like our own.
"I think showing how our planet is rare and special only increases the possibility of protecting it," Schwieterman said. "As far as we know, the Earth is the only planet in the universe that can sustain human life."
In addition to Schwieterman, Lyons, and Reinhard, the authors of the paper are Stephanie Olson of the University of Chicago and Chester E. Harman of Columbia University. This project was funded by the NASA Astrobiology Institute.
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