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The existence of habitable alien worlds has been a mainstay of popular culture for more than a century. In the 19th century, astronomers believed that Martians might be using canal-based transport links to traverse the red planet. Now, despite living in an age when scientists can study planets light-years from our own Solar System, most new research continues to diminish the chances of finding other worlds on which humans could live. The biggest stumbling block may be oxygen — human settlers would need a high oxygen atmosphere in which to breathe.

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