An alien welcomes tourists and invites them to buy charkeries, sweets and t-shirts in Baker, California. (Photo: Jefferson Graham, USA Today)

So maybe we are alone after all.

The number of planets in the universe that could support extraterrestrial life is much smaller than expected, astronomers announced this week in a statement. new study.

"Imagine a livable zone for complex living defined as a" safe zone "where it would be plausible to preserve ecosystems as rich as those found on Earth today," said the co – author of the study, Timothy Lyons, biogeochemist at the University of California-Riverside. "Our results indicate that complex ecosystems like ours can not exist in most areas of the livelihood zone as traditionally defined."

What's the problem? It has turned out that an accumulation of toxic gases in the atmosphere of most planets makes them unfit for complex life as we know it, the study said.

Traditionally, most planet searches capable of maintaining an extraterrestrial life focused on what scientists call the "habitable zone", which is where liquid water could exist on the surface of the planet. ;a planet. But, according to the new study, these planets could only maintain a very basic life, such as unicellular microbes – and not complex creatures like animals, which include everything from simple sponges to humans.

Thus, even though many planets have liquid water, many of them have a toxic atmosphere, the authors of the study said:

"To maintain liquid water at the edge of the conventional habitable zone, a planet would need tens of thousands of times more carbon dioxide than the Earth has today," said Edward Schwieterman, lead author of the study, NASA. "It's way beyond the levels known to be toxic to human and animal life on Earth."

As a result, only about one-third of known "habitable" planets, about 4,000 of which have been discovered by scientists, could retain complex life forms like us.

"This is the first time that the physiological limits of life on Earth have been used to predict the distribution of a complex life elsewhere in the universe," said Lyons.

Schwieterman said that "showing how rare and special our planet is is only reinforcing its protection, and to our knowledge, Earth is the only planet in the universe capable of sustaining human life."

The study was published Monday in the peer-reviewed journal Astrophysics.

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