AliensNarrowing the search for advanced life in the universe

Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life. 

In a new study, a UC Riverside–led team discovered that a buildup 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 the “habitable zone,” defined as the range of distances from a star warm enough that liquid water could exist on a planet’s surface. That description works for basic, single-celled microbes — but not for complex creatures like animals, which include everything 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 narrows the safe zone for complex life by at least half — and in some instances eliminates it altogether.

“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,” said 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, which sponsored the project.

“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,” Lyons explained. “Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”

UCR says that notes that using computer models to study atmospheric climate and photochemistry on a variety of planets, the team first considered carbon dioxide. Any scuba diver knows that too much of this gas in the body can be deadly. But planets too far from their host star require carbon dioxide — a potent greenhouse gas — to maintain temperatures above freezing. Earth included.

“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 working with Lyons. “That’s far beyond the levels known to be toxic to human and animal life on Earth.”