Rabu, 22 Oktober 2008

Dead’ planets might be livable after all

Astronomers have long talked about a "habitable zone" around a star as being a confined and predictable region where temperatures were not too cold, not too hot, so that a planet could retain liquid water and therefore support life as we know it. 
The zone may not be so fixed, it turns out. 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. 
A planet's midsection gets stretched out by its star's gravity so that its shape is slightly more like a cigar than a sphere. Some planets travel non-circular, or elongated paths around their stars. As such a world moves closer to the star, it stretches more, and when it moves farther away, the stretching decreases.
When a planet's orbit is particularly oblong, the stretching changes are so great that its interior warms up in a process called tidal heating. 
"It's basically the same effect as when you bend a paper clip, and it gets hot inside," said researcher Brian Jackson of the University of Arizona's Lunar and Planetary Laboratory. 
Jackson and colleagues created a computer model to simulate this effect on exoplanets, and found that the process could shift the range and distance of the "habitable zone" around a star in which planets would have the right temperatures needed to harbor life. 
"It could be that planets close to the edge of the habitable zone get way too much tidal heating, and they'd be too hot," Jackson told SPACE.com. It also could be that planets just beyond the outer edge, which according to previous models would be too cold, might undergo enough heating that their surfaces would be warm enough for life and water, Jackson said. 
Tidal heating could in fact affect many planets in the galaxy, because the oblong orbits that cause the phenomenon are quite common. 
"Most of the extrasolar planets we've found so far are in pretty elongated orbits, which is surprising because most of the planets in our solar system have orbits that are roughly circular," Jackson said. 
Scientists aren't sure why our solar system is unique in this way, but the difference could significantly affect the hunt for life beyond Earth.
In some cases it would suggest that it's going to be little bit harder, Jackson said, because worlds that looked habitable may experience too much tidal heating. On the other hand, some planets that were thought to be too cold might in fact be warmed up enough for life, and that might improve our odds. 
Tidal heating could further boost some planets' habitability by warming them enough to spur volcanism, which in turn drives plate tectonics, the process that recycles rock through a planet's surface layers. 
Plate tectonics is a definite boon for life, because stirring up the surface layers helps to regulate the amount of carbon dioxide in the atmosphere, since rock absorbs CO2 from the air. And having the perfect balance of carbon dioxide in the atmosphere helps a planet maintain that "just right" temperature range. 
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