As humans look farther into the universe and discover more
and more planets beyond the sun, many wonder how typical our own solar system
is. Often astronomers in the planet-hunting business say discoveries of
Earth-like worlds are just around the corner.
But a new study indicates our setup may be rare indeed.
A group of astronomers surveyed sun-like stars in the Orion
nebula open cluster and found that fewer than 10 percent have enough
surrounding dust to make Jupiter-sized
planets.
"We think that most stars in the galaxy are formed in
dense, Orion-like regions, so this implies that systems like ours may be the
exception rather than the rule," said researcher Joshua Eisner, an
astrophysicist at the University of California, Berkeley.
That's important because giant planets like Jupiter may be
instrumental in fostering life on rocky worlds like Earth.
Eisner and his team observed about 250 stars in the
million-year-old Orion Nebula, looking for dense disks of dust surrounding the
stars that could be forming planets. They found that only about 10 percent of
the stars emitted radiation in the frequency that would indicate they have
these proto-planetary disks of warm dust. And only 8 percent of the stars
surveyed had dust disks with masses greater than one-hundredth the mass of the
sun, a mass thought to be the lower limit for formation of Jupiter-sized
planets.
These findings seem to agree with what planet hunters are
finding so far when they use radial velocity studies to detect extrasolar
planets around other stars. (The radial velocity approach involves looking for
a wobble in a star's motion caused by the slight gravitational pull of an
orbiting planet.)
"The current numbers are suggesting 6 to 10 percent of
stars have Jupiter-sized planets, which is exactly consistent with our
findings," Eisner told SPACE.com.
The researchers will detail their findings in the Aug. 10
issue of the Astrophysical Journal.
Snapshot in time
Still, it's too soon to completely despair of finding the
universe filled with Jupiters around other suns.
Since the survey only looked at dust around the stars, and
would not have detected any already-formed planets, it could be that some of
those sun-like stars already had planets.
"Perhaps we're only detecting the stars that have not
formed planets yet," said John M. Carpenter, an astronomer at Caltech who
worked with Eisner on the Orion research. "Perhaps some other stars
already formed planets. It's only a snapshot in time and as you look at other
clusters at different ages you can build up a better picture."
Other scientists agree there are many unanswered questions
about solar
systems beyond our own.
"As the precision with which we can measure improves,
we find more planets," said Harvard planet hunter David Charbonneau, who
was not involved in the Orion study. "The rate of occurrence has gone up
since we started looking."
He said it's too soon to tell for sure whether Earth's
system is atypical, but studies that look at whether other stars have the raw
materials necessary to form solar systems like our own can help.
"Certainly knowing that there is enough stuff around
stars to make planets is a crucial step," he said.
Rare life?
If it turns out to be true that sun-like stars with
Jupiter-sized planets are rare, it may mean that extraterrestrial
life is rare as well.
Some scientists have suggested that having our own Jupiter
has been instrumental in forming life on Earth. For one thing, large planets
can protect smaller inner planets from being bombarded too heavily by space
rocks, which could crush any budding bits of life.
Plus, large planets could kick comets and asteroids out of
their orbits and onto paths toward the smaller terrestrial planets. These space
rocks could be delivery systems for organic materials and water.
"If you don't have a Jupiter it's kind of hard to build
a wet planet," Eisner said.
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