A new study narrows down the number of planets outside the solar system that have the right conditions to support life.

A team of researchers found that the buildup of toxic gases in the atmospheres of many planets make them unfit for complex life to develop and thrive.

"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," claimed Timothy Lyons, a professor of biogeochemistry at the University of California, Riverside and one of the authors of the study.

They published their findings in The Astrophysical Journal on Monday, June 10.

Search For Complex Alien Life In The Universe

The researchers studied the atmospheric climate and photochemistry of exoplanets by looking at computer models. They first considered carbon dioxide, a powerful greenhouse gas that aids in maintaining temperatures above freezing.

They discovered that simple animal life can only exist in exoplanets located in more than half of the traditional habitable zone of a star system. Toward the edges, an exoplanet would need extremely high levels of carbon dioxide to keep warm enough for liquid water to exist.

"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, a NASA Postdoctoral Program fellow and the lead author of the study. "That's far beyond the levels known to be toxic to human and animal life on Earth."

For higher order animals to survive, the actual safe zone shrinks even further.

No Habitable Zone Around Some Stars

Moreover, the researchers found that, in some planetary systems, there is no habitable zone. Cooler and dimmer stars emit intense ultraviolet radiation, which can result in high carbon monoxide concentration in a nearby planet's atmosphere. This is true for Proxima Centauri and TRAPPIST-1, two of the nearest stars to the sun.

Carbon monoxide is toxic: it binds to the hemoglobin which carries oxygen throughout the body. Even a tiny amount of carbon monoxide can seriously harm a human or an animal.

Christopher Reinhard, a former graduate student at the University of California, Riverside and one of the authors of the study, explained that the discoveries narrow down the number of exoplanets that scientists could probe to look for signs of complex alien life.