Even lifeless worlds can have oxygen-rich air

THINK you have spotted Earth’s faraway twin? Not so fast. Oxygenrich atmospheres, touted as almost sure signs of life on exoplanets, can exist on 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 planets in the habitable zone could have oxygen-rich air only if something was alive on the surface to keep pumping it out, as the highly reactive molecule would otherwise get quickly locked up in other compounds. But taking a closer look at the chemistry, Robin Wordsworth at the University of Chicago found that dead worlds made largely of water can have lots of oxygen in their air (arxiv.org/abs/1403.2713). 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. The work suggests that alienhunters will need to look more carefully at exoplanet signals, such as the exact ratio of oxygen to nitrogen. This makes the task more difficult, says Jack O’MalleyJames at the University of St Andrews in the UK. But finding the right mix of chemical clues will increase confidence that we are seeing evidence of life, he says.