Finding signs of life on planets in other solar systems could be unexpectedly tough, because they may not have the oxygen-rich atmosphere usually taken as the signature of life.
Earth was oxygen-poor when photosynthesising bacteria made their debut at least 2.7 billion years ago. Even then oxygen concentrations crept up only slowly until 2.4 billion years ago, when they jumped by a factor of 1000. Now a model developed by Colin Goldblatt and colleagues at the University of East Anglia in Norwich, UK, has shown that the jump occurred after oxygen reached a critical level that allowed ozone to form, blocking the sun's ultraviolet light and suppressing the destruction of oxygen (Nature, vol 443, p 683).
It needn't be that way. A planet teeming with life might tick along in a stable low-oxygen state for billions of years without reaching the critical point, Goldblatt says.
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