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Think you've spotted Earth's faraway twin? Not so fast. Oxygen-rich atmospheres, touted as almost sure signs of life on habitable exoplanets, can also exist around dead worlds.

More than 1750 exoplanets have been found so far, including several in the habitable zone – the region around a star where liquid water can exist. Next-generation telescopes should be able to "sniff" exoplanet atmospheres, revealing clues to their make-up.

Previous research suggested that oxygen could build up on lifeless planets only if they are outside of or on the edge of the habitable zone. But by definition, the best chances for finding aliens are on worlds squarely inside the life-friendly zone, and it was thought that these planets could have oxygen-rich air only if something was alive on the surface to keep pumping it out. That's because chemical modelling suggested that in the warmer atmospheres of these habitable planets, the highly reactive oxygen molecule would quickly get locked up in other compounds.

But taking a closer look at the chemistry, Robin Wordsworth at the University of Chicago found that, even in the middle of the habitable zone, dead worlds made largely of water can have lots of oxygen in their air. Ultraviolet light from the planet's star can split molecules of water vapour, with lighter hydrogen drifting into space while oxygen is left behind. The same process could happen on life-free rocky planets if they have low levels of nitrogen or argon, gasses that keep water vapour out of the air and halt oxygen build-up.

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