The number of planets that might provide a favorable atmosphere for alien life is much smaller than it had been previously thought, astronomers revealed in a new study published in the peer-reviewed Astrophysical Journal.

“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,” said the study co-author Timothy Lyons, a biogeochemist at the University of California–Riverside.

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

According to the study, an increase level of toxic gases in the atmospheres of most planets makes them unhealthy for complex life, saying that these planets can only be able to sustain very basic life such as single-celled microbes.

"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 NASA's Edward Schwieterman, the study’s lead author.

"That's far beyond the levels known to be toxic to human and animal life on Earth,” he noted.

“Showing how rare and special our planet is only enhances the case for protecting it. As far as we know, Earth is the only planet in the universe that can sustain human life.”