EARTH-LIKE planets may be stuck in a catch-22 that makes the existence of life very unlikely. They would need stable, warm climates for complex life to evolve, but without complex life their climates may never stabilise.
Earth has a carbon-silicate cycle that acts like a thermostat over geological timescales. High temperatures increase weathering of silicate rocks, and this sucks carbon dioxide out of the atmosphere and into the oceans – a process aided by plants. Lower CO2 levels cause temperatures to fall, leading to widespread glaciation and less silicate weathering. Levels of atmospheric CO2 later rise, leading to warmer temperatures.
"A natural question then is how efficiently does [this cycle] work on other planets," says Kristen Menou of the University of Toronto in Canada.
Menou considered Earth-like extrasolar planets that lie at the far reaches of their host star's habitable zone, which would receive low levels of sunlight. In the absence of complex plants, Menou's analysis shows that these planets would remain glaciated much of the time and only transiently enter a state with enough liquid water for life to take hold (http://ift.tt/1pUM6PM).
This article appeared in print under the headline "Extrasolar planets face life paradox"
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