Human evolution; needs habitable planets (Introduction)

by David Turell , Tuesday, June 11, 2019, 01:18 (1991 days ago) @ David Turell

This study says few are likely to exist besides us:

https://phys.org/news/2019-06-narrows-advanced-life-universe.html

"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.

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"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,

"'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."

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"'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.

"What is more, no safe zone at all exists for certain stars, including two of the sun's nearest neighbors, Proxima Centauri and TRAPPIST-1. The type and intensity of ultraviolet radiation that these cooler, dimmer stars emit can lead to high concentrations of carbon monoxide, another deadly gas. Carbon monoxide binds to hemoglobin in animal blood—the compound that transports oxygen through the body. Even small amounts of it can cause the death of body cells due to lack of oxygen.

"Carbon monoxide cannot accumulate on Earth because our hotter, brighter sun drives chemical reactions in the atmosphere that destroy it quickly. Although the team concluded recently 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."

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"'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.'"

Comment: I'm not surprised. Our planet has many fine-tuned and undoubtedly unique features. I suspect we are alone