[tt] Science Daily: Solar System Is Pretty Special, According To New Computer Simulation

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Solar System Is Pretty Special, According To New Computer Simulation
http://www.sciencedaily.com/releases/2008/08/080807144236.htm

ScienceDaily (Aug. 8, 2008) -- Prevailing theoretical models
attempting to explain the formation of the solar system have assumed
it to be average in every way. Now a new study by Northwestern
University astronomers, using recent data from the 300 exoplanets
discovered orbiting other stars, turns that view on its head.

The solar system, it turns out, is pretty special indeed. The study
illustrates that if early conditions had been just slightly
different, very unpleasant things could have happened -- like
planets being thrown into the sun or jettisoned into deep space.

Using large-scale computer simulations, the Northwestern researchers
are the first to model the formation of planetary systems from
beginning to end, starting with the generic disk of gas and dust
that is left behind after the formation of the central star and
ending with a full planetary system. Because of computing
limitations, earlier models provided only brief glimpses of the
process.

The researchers ran more than a hundred simulations, and the results
show that the average planetary system's origin was full of violence
and drama but that the formation of something like our solar system
required conditions to be "just right."

The study was recently published in the journal Science.

Before the discovery in the early 1990s of the first planets outside
the solar system, our system's nine (now eight) planets were the
only ones known to us. This limited the planetary formation models,
and astronomers had no reason to think the solar system unusual.

"But we now know that these other planetary systems don't look like
the solar system at all," said Frederic A. Rasio, a theoretical
astrophysicist and professor of physics and astronomy in
Northwestern's Weinberg College of Arts and Sciences. He is senior
author of the Science paper.

"The shapes of the exoplanets' orbits are elongated, not nice and
circular. Planets are not where we expect them to be. Many giant
planets similar to Jupiter, known as 'hot Jupiters,' are so close to
the star they have orbits of mere days. Clearly we needed to start
fresh in explaining planetary formation and this greater variety of
planets we now see."

Using the wealth of exoplanet data collected during the last 15
years, Rasio and his colleagues have been working to understand
planet formation in a much broader sense than was possible
previously. Modeling an entire planetary system -- the varied
physical phenomena associated with gas, gravity and grains of
material, on such a variety of scales -- was a daunting challenge.

The work required very powerful computers. The researchers also had
to judiciously decide what information was important and what was
not, so as to speed up the calculations. They decided to follow the
growth of planets, the gravitational interaction between planets,
and the whole planetary system in its entire spatial extent. They
chose not to follow the gas disk's fluid dynamics in fine detail,
but rather more generally. As a result, they were able to run
simulations spanning a planetary system's entire formation.

The simulations suggest that an average planetary system's origin is
extremely dramatic. The gas disk that gives birth to the planets
also pushes them mercilessly toward the central star, where they
crowd together or are engulfed. Among the growing planets, there is
cut-throat competition for gas, a chaotic process that produces a
rich variety of planet masses.

Also, as the planets approach each other, they frequently lock into
dynamical resonances that drive the orbits of all participants to be
increasingly elongated. Such a gravitational embrace often results
in a slingshot encounter that flings the planets elsewhere in the
system; occasionally, one is ejected into deep space. Despite its
best efforts to kill its offspring, the gas disk eventually is
consumed and dissipates, and a young planetary system emerges.

"Such a turbulent history would seem to leave little room for the
sedate solar system, and our simulations show exactly that," said
Rasio. "Conditions must be just right for the solar system to
emerge."

Too massive a gas disk, for example, and planet formation is an
anarchic mess, producing "hot Jupiters" and noncircular orbits
galore. Too low-mass a disk, and nothing bigger than Neptune -- an
"ice giant" with only a small amount of gas -- will grow.

"We now better understand the process of planet formation and can
explain the properties of the strange exoplanets we've observed,"
said Rasio. "We also know that the solar system is special and
understand at some level what makes it special."

"The solar system had to be born under just the right conditions to
become this quiet place we see. The vast majority of other planetary
systems didn't have these special properties at birth and became
something very different."

###

In addition to Rasio, other authors of the Science paper are Edward
W. Thommes, an adjunct professor at the University of Guelph in
Ontario, former postdoctoral fellow at Northwestern and lead author,
and Soko Matsumura, a postdoctoral fellow at Northwestern.

The computer simulations were performed on a supercomputing cluster
operated by Northwestern's Theoretical Astrophysics Group and
partially funded by a Major Research Instrumentation grant from the
National Science Foundation (NSF). Rasio's research group on
exoplanets also is funded by a grant from the NSF Division of
Astronomy.
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Journal reference:
1. . Gas Disks to Gas Giants: Simulating the Birth of Planetary
Systems. Science, August 8, 2008

Adapted from materials provided by Northwestern University, via
EurekAlert!, a service of AAAS.