New study dramatically reduces the search for advanced life in the universe

Scientists may need to rethink their estimates of the number of planets outside our solar system that could harbor a rich diversity of lives.

In a new study, a team led by UC Riverside discovered that an accumulation of toxic gases in the atmosphere of most planets makes them unfit for complex life as we know it.

Traditionally, the quest for extraterrestrial life has largely focused on what scientists call the "habitable zone", defined as the distance between a star hot enough for liquid water to exist on the surface of the planet. . This description works for basic unicellular microbes, but not for complex creatures like animals, which include everything from simple sponges to humans.

The work of the team, published today in The Astrophysical Journal, shows that taking into account the predicted levels of certain toxic gases reduces the safety zone for complex life by at least half, or even eliminates it in some cases.

"This is the first time that we consider that the physiological limits of life on Earth can predict the distribution of a complex life elsewhere in the universe," said Timothy Lyons, one of the most -authors of the study, distinguished professor of biogeochemistry at the Department of Earth Sciences and Planets and director of the Center for Astrobiology of Alternative Lands, who sponsored the project.

"Imagine a" living zone for complex life "defined as a safe zone where it would be plausible to preserve ecosystems as rich as those we find on Earth today," said Lyons. "Our results indicate that complex ecosystems like ours can not exist in most areas of the livelihood zone as traditionally defined."

Using computer models to study atmospheric climate and photochemistry on various planets, the team first became interested in carbon dioxide. Any underwater diver knows that too much of this gas in the body can be deadly. But planets too far from their host star need carbon dioxide, a potent greenhouse gas, to keep their temperature above zero. Earth included.

"To maintain 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, author Principal Investigator and NASA Postdoctoral Fellow working with Lyon. "It's way beyond the levels known to be toxic to human and animal life on Earth."

The new study concludes that the only toxicity of carbon dioxide limits simple animal life to more than half of the traditional livelihood zone. For humans and other higher order animals, which are more sensitive, the safety zone is reduced to less than one-third of this area.

In addition, there is no safety zone for some stars, including two of the sun's closest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation emitted by these colder and weaker stars can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide binds to the hemoglobin in animal blood – the compound that carries oxygen through the body. Even small amounts can cause death of body cells due to lack of oxygen.

Carbon monoxide can not accumulate on Earth because our warmer and brighter sun causes chemical reactions in the atmosphere that destroy it quickly. Although the team has recently concluded that microbial biospheres may flourish on a carbon monoxide rich planet, Schwieterman pointed out that "these sites would certainly not be good places for human or animal life such as we know it on Earth ".

Scientists have confirmed that nearly 4,000 planets revolve around stars other than the sun, but none of them can be visited in person. They are just too far away. The closest is Proxima Centauri b, which would take 54,400 years to reach the current spacecraft. The use of telescopes to detect the abundance of certain gases in their atmosphere is one of the only ways to study these so-called exoplanets.

"Our findings provide a way of deciding which of these myriad planets we need to observe in more detail," said Christopher Reinhard, a former UCR graduate student, now assistant professor at the Georgia Institute of Technology, co-author of this study, and co-author-Leader of the Alternative Earths team. "We could identify otherwise habitable planets with levels of carbon dioxide or carbon monoxide that are probably too high to support a complex life."

Earlier work by the team has already documented the next generation of space missions, such as NASA's proposed living exoplanet observatory. For example, since oxygen is essential to complex life on Earth and can be detected remotely, the team has studied its potential in atmospheres of different planets.

Apart from the Earth, no planet in our solar system hosts a life that can be characterized from a distance. According to Schwieterman, if life exists elsewhere in the solar system, it lies below a rocky or icy surface. Exoplanets could be our best hope to find habitable worlds closer to ours.

"I think showing how rare and special our planet is is just reinforcing its protection," Schwieterman said. "As far as we know, the Earth is the only planet in the universe capable of sustaining human life."