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Does oxygen necessarily mean aliens?

Astrobiologists find that the presence of oxygen in a planet's atmosphere may not necessarily mean that life exists there.

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An artist drew this representation of planets Kepler-62e and -f. Scientists using NASA's Kepler telescope found two distant planets that are in the right place and are the right size for potential life. The larger planet in the left corner, f, is somewhat covered by ice and is is farther from their shared star. The planet below it is e, which is slightly warmer, has clouds, and may be a water world.

Harvard Smithsonian Center for Astrophysics/AP/File

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Scientists and E.T. enthusiasts may have to rethink an allegedly telltale sign that a planet has life.

The presence of oxygen, specifically O2, in a planet's atmosphere has long been thought to be a near-certain signal that there are, or at least were, living organisms engaging in photosynthesis on the planet. But new research suggests that oxygen can exist in large quantities without being produced by living things.

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A study published Thursday in Scientific Reports found that some planets could have "abiotic" oxygen, produced through a a photocatalytic reaction of titanium oxide.

Recommended:What makes a planet livable? Five things scientists look for.

For scientists hunting for extraterrestrial life, this abiotic oxygen could create a false positive, the researchers caution.

Atmospheric oxygen, like water and complex organic molecules, have long been considered biomarkers, that is, signals that life is present.

"To search for life on extrasolar planets through astronomical observation, we need to combine the knowledge from various scientific fields and to promote astrobiology researches to establish the decisive signs of life,” study author and astrobiology professor Norio Narita said in a news release.

"Although oxygen is still one of possible biomarkers," said Dr. Narita, the new results suggest that scientists should "look for new biomarkers besides oxygen."

Water has been a popular biomarker, especially as it has been found in moons in our solar system and in atmospheres around other Earth-like planets.

This isn't the first time scientists have debunked a signifier. In 2013, astronomers found that a planet could be surrounded by atmospheric gases that suggested life, but that those gases too could be a false indication. "Beware the habitable-planet poseur," wrote the Christian Science Monitor's Pete Spotts.

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It's all part of the evolving understanding of planetary processes and what they mean for extraterrestrial life. 

NASA isn't discouraged. Within 20 to 30 years, says NASA chief scientist Ellen Stofan, scientists will find clear evidence of life "out there."