Scientists’ search for aliens has become more focused on a smaller number of planets after making a discovery about the composition of most planets’ atmospheres. Typically, alien hunting experts have been analysing planets which are in the habitable zone of their host star – an region in space where it is neither too cold nor too warm for life to exist. However, experts from the University of California Riverside (UCR) believe other scientists have failed to take into account a build up of toxic gasses within a planet’s atmosphere which would not allow complex life to evolve.
For example, by using computer models the researchers found that any planet on the outer edge of the habitable zone with liquid on the surface would require carbon dioxide – a greenhouse gas – levels thousands of times that of Earth’s to maintain liquid and not have it freeze.
Edward Schwieterman, lead author of the study published in The Astrophysical Journal, and a NASA Postdoctoral Program, said: “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.
“That’s far beyond the levels known to be toxic to human and animal life on Earth.”
Another example comes from our solar system’s two closest alien stars – Proxima Centauri and TRAPPIST-1.
The intense ultraviolet radiation from these stars would batter any planets in their habitable zones, leading to a build up of carbon monoxide, which is poisonous.
Timothy Lyons, one of the study’s co-authors, a distinguished professor of biogeochemistry in UCR’s Department of Earth and Planetary Sciences, and director of the Alternative Earths Astrobiology Center, said: “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.
“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.
“Our results indicate that complex ecosystems like ours cannot exist in most regions of the habitable zone as traditionally defined.”
Mr Schwieterman concluded: “I think 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.”