By Lee Bowman
Scripps Howard News Service
March 29, 2005
Although there are examples of some microbes surviving in ice or extreme heat, most known life forms require liquid water, which sets the definition of habitable zone as the region around a sun where liquid water can remain on the surface of a planet orbiting the star.
If a planet is too close to its sun, any water it has will boil away into space; too far away, the water will freeze.
Our own sun's habitable zone is considered to be from slightly closer than Earth's orbit to about 1.6 times Earth's average orbit (about 93 million miles). Other stars, depending on their size, type and brightness, have varying habitable zones.
But astronomers know that stars get brighter as they age, pushing their habitable zones farther out, and possibly bringing warmth and life to plants that had been inhospitable before.
In a new study, researchers from NASA and several French institutions tried to quantify how those zones might change with different types of stars beyond our own solar system, focusing on stars with the same mass as our sun as well as those up to twice as large.
"Our results indicate that searches for life-giving worlds outside our own solar system should include planets around old stars," said Bruno Lopez, lead author of the study being published in an upcoming issue of the Astrophysical Journal and a researcher at the Cote d'Azur Observatory in Nice, France.
Earth's fossil record, along with a geological record of carbon that may have been left behind by the earliest life, offers the only model timetable for the emergence of life, roughly a half billion to a billion years after a planet becomes habitable.
This zone expands as stars reach old age, use up their hydrogen fuel and enlarge to become red giants, a transit of the habitable zone that lasts from a few hundred million years to a couple of billion years, according to the researchers.
The team calculates that there are about 150 sub-giant or red giant stars within 100 light years of Earth. Any planets that might be found orbiting those mature suns would be good candidates to have their atmospheres scanned in order to observe any signatures of life in the form of light emitted by molecules that develop during biological processes. Scientists reported just last week their first success using a NASA infrared telescope to capture light from giant extra-solar planets.
In our own solar system, the sun is expected to become a red giant in another 4 billion to 5 billion years, expanding beyond the orbits of Mercury, Venus and probably Earth, but bringing Mars to the inner edge of the habitable zone. Mars now is on the outer edge where its weak gravity leaves an atmosphere too thin to hold heat that allows water to stay liquid for long, although scientists believe it may have once been warmer.
"Mars will be in the habitable zone for a couple of billion years, so Martian life may get a second chance," said William Danchi, a researcher at Goddard Space Flight Center in Greenbelt, Md.
The scientists note that there's a reasonable probability that some Earth bacteria may have been transported to Mars by meteors that chipped away at our planet.
"As the sun grows ever brighter and Earth overheats, terrestrial life might hitch a ride on a meteor and find a new home on Mars," Danchi said.
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