Swansong Earth: Refuges for Life

by PAUL GILSTER on OCTOBER 31, 2012

As we begin to identify planets in the habitable zone of their stars, the larger issue becomes what fraction of stars have such planets. This is eta-Earth (?Earth), the percentage of Sun-like stars with Earth-like planets in the habitable zone, a figure we can gradually home in on as statistical surveys like Kepler continue to churn. Right now the estimate depends on whom you talk to, with figures ranging from 1.4-2.7% (Catanzarite & Shao, 2011) to 42% for red dwarf stars (Bonfils et al., 2011). One thing I haven’t seen discussed much is the question of when planets cease being habitable, including what we can call the phases of habitability on a given world.

Jack T. O’Malley-James (University of St Andrews, UK) and colleagues have gone to work on the question in a new paper slated for publication in the International Journal of Astrobiology. The researchers note that life emerged on Earth 3.8 billion years ago and perhaps somewhat earlier. The key point is that unicellular organisms were found on our planet at least 2.5 billion years before fossil evidence of multicellular life appears. And most animal phyla do not appear as body fossils until 530 million years ago, during the period known as the ‘Cambrian explosion.’

Read more: Swansong Earth: Refuges for Life — Centauri Dreams.

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