The habitable zone for complex life around many stars could be much smaller than previously thought once the concentrations of carbon monoxide and carbon dioxide on planets is considered. That is the conclusion of astrobiologists in the US, who say that high concentrations of these gases could completely preclude the existence of life on planets orbiting some stars.

The search for extraterrestrial life often focuses on what is known as the “habitable zone” of stars. This is commonly defined as the range of distances from a host star warm enough for liquid water, a key requirement for life, to exist on a planet’s surface. However, according to Edward Schwieterman, at the University of California, Riverside, and his colleagues that description works for basic, single-celled microbes, but not for more complex creatures – everything from simple sponges to humans.

They point out that substantially more carbon dioxide than present in Earth’s atmosphere is needed to maintain suitable temperatures over much of the traditionally-defined habitable zone. At the outer edge of the zone, concentrations of several bars would be required, they say, yet most complex aerobic life on Earth is limited by concentrations of just fractions of a bar.

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