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>A stroll through Earth's past gives astronomers vital clues of what to expect when they start finding terrestrial-size exoplanets
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A stroll through Earth's past gives astronomers vital clues of what to expect when they start finding terrestrial-size exoplanets
THE HOLY GRAIL in the search for extrasolar plan-ets is finding one that resembles our home planet. But it isn't enough to search for spectroscopic signatures of our modern Earth. The planet has profoundly changed over its 4.56 billion years. Temperatures ranged from steaming hot to global deep freezes that covered the equator with ice sheets. The atmosphere contained virtually no free oxygen until about 2.4 billion years ago, and probably didn't approach the modern 21% concentration until plants and animals spread across the land less than half a billion years ago.
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