Maybe we are alone after all: Planets that could sustain alien life much rarer than thought Doyle Rice USA TODAY Published 3:13 PM EDT Jun 11, 2019 An alien greets tourists and invites them to buy jerky, candy and T-shirts in Baker, California. Jefferson Graham, USA TODAY So, maybe we are alone after all. The number of planets in the universe that could sustain alien life is much smaller than had been thought, astronomers announced this week in a new study. “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 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." What's the problem? It turns out that a buildup of toxic gases in the atmospheres of most planets makes them unfit for complex life as we know it, the study said. Traditionally, most of the search for planets capable of sustaining alien life has focused on what scientists call the “habitable zone,” which is where liquid water could exist on a planet’s surface. But, according to the new study, these planets would only be able to sustain very basic life, such as single-celled microbes – not complex creatures like animals, which include everything from simple sponges to humans. So even though lots of planets may have liquid water, many of them have toxic atmospheres, the study authors 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," 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." Therefore, only about a third of the known "habitable" planets, of which scientists have discovered about 4,000, could sustain complex life forms like us. “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 Lyons. Schwieterman said that "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.” The study was published Monday in the peer-reviewed Astrophysical Journal.