October 27, 2008 | David F. Coppedge

Habitable Zones Are Not Forever

A new realization has broken on the astrobiological community: planetary habitable zones have no fences.  Michael Sherber wrote for Astrobiology Magazine (see Space.com) that planets around low-mass stars tend to be pulled out of the habitable zone toward the star.  They have just a billion years before migration can pull them in and cook them.  “Planets around small mass stars may only have a billion-year window during which life can form.”  He did not indicate whether that life would be very happy, though, knowing a fiery hell awaited it.  “Habitability is not a permanent property of a planet,” the article said.
    Rory Barnes (U of Arizona) who thought about this, wondered if it could be a test of the Gaia Hypothesis – something he termed “a grand picture of evolution.”  Maybe the lifeforms could adapt as the planet migrates inward by altering the planet’s climate and geochemistry.  Maybe we could even learn “how life mitigates disasters and adapts to climate change,” he said.
    Meanwhile, Clara Moskowitz at Space.com had more optimistic news.  Some planets once thought inhospitable might actually be able to support life.  “The [habitable] zone may not be so fixed, it turns out,” she said.  “Some extrasolar planets that one might assume are too cold to host life could in fact be made habitable by a squishing effect from their stars, a new study found.”  If the planet has an oblong orbit, the tidal heating would heat it up.  Maybe this could melt the ice of a planet outside the zone and give it hope for life.  Maybe it could start volcanoes and plate tectonics.  That’s a lot of maybes.  One thing we know: it all comes together just right where we live: “Plate tectonics is a definite boon for life,” she said, “because stirring up the surface layers helps to regulate the amount of carbon dioxide in the atmosphere, since rock absorbs CO2 from the air.”  That “perfect balance” helps a planet maintain that “just right” temperature range.

Quiz question: what is an evolutionist’s favorite word?  Oh, you want a free hint?  Maybe, baby.

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