June 11, 2019 | David F. Coppedge

Cut the Habitable Worlds by Half (or More)

Astrobiologists figure that most earth-like planets give off toxic gases.

A press release from UC Riverside announces, “New study dramatically narrows the search for advanced life in the universe.” Reporter Jules Bernstein adds, “Toxic gases limit the types of life we could find on habitable worlds.” The implications are that “Scientists may need to rethink their estimates for how many planets outside our solar system could host a rich diversity of life.”

A planet cannot be considered habitable simply for orbiting in the so-called “habitable zone” (HZ) where liquid water can exist. UCR researchers have come to realize that toxic gases can build up over time on many exoplanets, even if they are safe inside the habitable zone. The study, published in the prestigious 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.

Why is that? Timothy Lyons and team figured that, except within a narrow zone within the traditional habitable zone, an exoplanet would require so much carbon dioxide (CO2), it would poison complex life.

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.

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 one third of that area.

Not only that, some of the most promising candidates for earthlike exoplanets (TRAPPIST-1 and Proxima Centauri b) have stars that would generate poisonous carbon monoxide (CO) in abundance. For complex life, the planet needs free oxygen, but not too much carbon dioxide or carbon monoxide. These three molecules must be finely balanced. The common M-dwarf stars may be the last places to look for intelligent life. The paper in ApJ concludes,

Our results have a number of important implications for the search for exoplanet biosignatures and complex life beyond our solar system. For example, our predictions of a more limited zone for complex life place constraints on the planetary environments suitable for the evolution of intelligence, if it requires free O2 and limited concentrations of CO2, CO, and other potentially toxic trace gases. One implication is that we may not expect to find remotely detectable signs of intelligent life (“technosignatures”) on planets orbiting late M dwarfs or on potentially habitable planets near the outer edge of their HZs. These CO2 and CO limits should be considered in future targeted SETI searches. The possible importance of photochemistry in creating environments conducive to complex and intelligent life further suggests a strong need for stellar UV characterization (e.g., France et al. 2016; Loyd et al. 2016; Youngblood et al. 2016) not only for biosignature prediction and assessment but also for SETI target prioritization….

We suggest that the expected physiological impacts of high CO2, CO, and other gases possibly toxic for complex life should be considered in attempts to search for biological complexity beyond our solar system.

In short, the amount of ultraviolet radiation produced by the host star must be considered in estimates of habitability. UV light will have profound impacts on the ratios of molecules in the atmospheres of exoplanets.

These findings must be discouraging for NASA’s Astrobiology Project that funded the study. Laura Geggel at Live Science says that the study “dramatically reduces the number of worlds where scientists will have the best luck finding ET.”

There is one bright side, according to the UCR press release:

“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.”

The book Spacecraft Earth by Richter and Coppedge lists 15 requirements for habitability of complex life. The new findings add more information to one factor in the list, the “ultraviolet habitable zone.” Not only must the planet’s atmosphere filter out UV light, the host star must not produce so much UV as to swamp the atmosphere with deadly photochemical molecules.

Dr Richter’s book examines many amazing finely-tuned “coincidences” that make our planet habitable

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