From: LARRY KLAES (ljk4@msn.com)
Date: Fri Nov 29 2002 - 19:26:15 PST
----- Original Message -----
From: NASA Jet Propulsion Laboratory
Sent: Friday, November 29, 2002 7:10 PM
To: ljk4@msn.com
Subject: Jupiter-Like Planets Formed In Hundreds - Not Millions - Of Years, Study Shows
University of Washington
Seattle, Washington
FOR IMMEDIATE RELEASE: Nov. 28, 2002
FROM: Vince Stricherz, 206-543-2580, vinces@u.washington.edu
Jupiter-like planets formed in hundreds -- not millions --
of years, study shows
An accepted assumption in astrophysics holds that it takes
more than 1 million years for gas giant planets such as
Jupiter and Saturn to form from the cosmic debris circling
a young star. But new research suggests such planets form
in a dramatically shorter period, as little as a few
hundred years.
The forming planets have to be able to survive the effects
of nearby stars burning brightly, heating and dispersing
the gases that accumulate around the giant planets. If
the process takes too long, the gases will be dissipated
by the radiation from those stars, said University of
Washington astrophysicist Thomas R. Quinn.
"If a gas giant planet can't form quickly, it probably
won't form at all," he said.
The standard model of planet formation holds that the
spinning disk of matter, called a protoplanetary disk,
that surrounds a young star gradually congeals into
masses that form the cores of planets. That process was
thought to take a million years or so, and then the
giants gradually accumulate their large gaseous envelopes
over perhaps another 1 million to 10 million years.
But the new research, culled from a much-refined
mathematical model, suggests that the protoplanetary disk
begins to fragment after just a few spins around its star.
As the disk fragments, clusters of matter begin to form
quickly and immediately start to draw in the gases that
form vapor shrouds around gas giants.
"If these planets can't form quickly, then they should
be a relatively rare phenomenon, whereas if they form
according to this mechanism they should be a relatively
common phenomenon," said Quinn, a UW research assistant
astronomy professor.
The existence of gas giant planets, it turns out, seems
to be fairly common. Since the mid-1990s, researchers
have discovered more than 100 planets, generally from
the mass of Jupiter to 10 times that size, orbiting
stars outside the solar system. Those planets were
deduced by their gravitational effect on their parent
stars, and their discovery lends credence to the new
research, Quinn said.
Lucio Mayer, a former UW post-doctoral researcher who
recently joined the University of Zurich, is lead author
of a paper detailing the work, published in the Nov. 29
edition of Science. Besides Quinn, co-authors are James
Wadsley of McMaster University, Hamilton, Ontario, Canada,
and Joachim Stadel at the University of Victoria, British
Columbia, Canada. Their work is supported by grants from
the National Science Foundation and the National
Aeronautics and Space Administration's Astrobiology
Institute.
Since the early 1950s, some scientists have entertained
the notion that gas giant planets were formed quickly.
However, the model, using a specialized fluid dynamics
simulation, had never been refined enough to show what
it does now. The Mayer-Quinn team spent the better part
of two years refining calculations and plugging them into
the model to show what would happen to a protoplanetary
disk over a longer time.
"The main criticism people had of this model was that it
wasn't quite ready yet," Quinn said. "Nobody was making
any predictions out of it, but here we are making
predictions out of it."
The new model explains why two other giant planets in our
system, Uranus and Neptune, don't have gas envelopes like
Jupiter and Saturn, Quinn said. At the time those planets
were being formed, the solar system was part of a star
cluster. The outer planets of Uranus and Neptune were too
close to a nearby star -- one that has since migrated
away -- and therefore lost whatever gas envelopes they
might have accumulated.
Neither the new model nor the standard model accounts for
why most of the gas giant planets found outside the solar
system are much nearer their suns than are Jupiter and
Saturn, Quinn said. The most common belief currently is
that the planets formed farther away from their stars
and then migrated inward to the positions where they
have been discovered.
The new model also doesn't account for the formation of
terrestrial planets, like Earth and Mars, near our sun.
But Quinn suspects that perhaps the smaller terrestrial
planets were formed over longer periods by processes
described by the standard planet-formation model, while
the new model explains how the larger gas giants came to
be.
"That's my bet at the moment," he said.
###
For more information, contact Quinn at (206) 685-9009 or
trq@astro.washington.edu, or Mayer at (011) 41-1-6355740
or lucio@physik.unizh.ch
IMAGE CAPTION:
[http://www.washington.edu/newsroom/news/images/disk-hr.jpg (23KB)]
A computer simulation shows how a protoplanetary disk
surrounding a young star begins, in a relatively short time,
to fragment and form gas giant planets with stable orbits.
(Photo credit: Mayer, Quinn, Wadsley, Stadel)
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