SIGHTINGS



Extrasolar Planet
Discoveries Raise Questions
About Life

By Kenneth Silber - Staff Writer
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12-26-99
 
 
 
As a wealth of new data flooded in about planets beyond our solar system in 1999, scientists gained a clearer sense of what they don,t know -- particularly about whether extrasolar planets might harbor life.
 
A dozen extrasolar planets were discovered in 1999, bringing the total number to 28 and confirming that planets are not an extreme rarity in the universe. The year saw the first detection of a planet crossing in front of a star, and the first discovery of a multi-planet system around a sunlike star.
 
All of the planets that have been discovered are gas giants, and many of them are "hot Jupiters," gas giants that travel around their stars in close, swift orbits. "What's becoming clear is that hot Jupiters are reasonably common," says John Bally, a professor of astronomy at the University of Colorado.
 
Hot Jupiters are generally not considered likely abodes for life. But their existence, notes Bally, may be only the "tip of the iceberg." Present-day instruments are unable to detect any smaller, Earthlike planets that may exist, and even gas giants are hard to detect if they travel in orbits distant from their stars.
 
Some gas giants are found in the "habitable zone," orbiting at roughly the same distance from their stars as the Earth is from the sun. But their presence there carries mixed implications for the possibility of life.
 
"Giant gas planets will scatter small planets out of their path, very much like cars are going to scatter flies out of their path," says William Borucki, a research scientist at NASA's Ames Research Center. Thus, he says giant gas planets in the habitable zone "preclude habitable planets around those stars."
 
"It's interesting to ask whether a solar system like ours is a requirement for technological life." Chris Chyba, SETI Institute
 
On the other hand, such planets might have moons that are habitable. "If these Jupiters are in stable orbits, which they seem to be, then although the planet might not be suitable for life, any moons that might orbit around them might be," says Bally, who adds that this is "pure speculation."
 
Currently, it is not known whether any extrasolar planets have moons, or whether such moons would be large enough to maintain thick atmospheres that provide a shield against a gas giant's radiation.
 
How similar to our own solar system?
 
A further question is whether gas giants are common in orbits far from their stars.
 
"Under the most propitious situation, the giant planets form and stay out a long distance," says Borucki, noting that Jupiter helps divert comets away from Earth and our inner solar system. However, he adds, it may be that numerous Earthlike planets can form in a solar system if there are no gas giants there at all.
 
Earthlike planets or moons, moreover, may not be the crucial conditions for life. Even if all life requires liquid water, "you probably don,t need solar systems like ours," says Chris Chyba, holder of the SETI Institute's Carl Sagan Chair for the Study of Life in the Universe.
 
There may be extrasolar worlds similar to Jupiter's moon Europa, which appears to have an ocean beneath layers of ice, Chyba points out. "It's clear from what we've learned about life on Earth that you can have ecosystems that flourish underground as long as you have liquid water."
 
He adds: "It's interesting to ask whether a solar system like ours is a requirement for technological life. Can you imagine a technically intelligent species evolving underneath a kilometer of ice on a world like Europa? That's much harder to envision, although our understanding here is very limited."


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