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Posted by baalke on May 23, 2007, 11:52 am
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PIO Contact: Rebecca Johnson
McDonald Observatory
The University of Texas at Austin
512-4715-6763
rjohnson@astro.as.utexas.edu
FOR IMMEDIATE RELEASE
May 23, 2007
TEXAS ASTRONOMERS DISCOVER MULTI-PLANET SYSTEM
AROUND UNEXPECTED STAR; MAY ALTER PLANET-FORMATION THEORIES
AUSTIN, Texas -- University of Texas at Austin astronomers
William Cochran and Michael Endl, working with graduate
students Robert Wittenmyer and Jacob Bean, have used the
9=2E2-meter Hobby-Eberly Telescope (HET) at McDonald
Observatory to discover a system of two Jupiter-like planets
orbiting a star whose composition might seem to rule out
planet formation. This NASA-funded study has implications
for theories of planet formation.
Cochran and Endl have been monitoring the star, HD 155358,
since 2001 using the High Resolution Spectrograph on HET.
Their measurements of its "radial velocity," or motion
toward and away from Earth, show that the star has a wobble
in its motion, which is caused by unseen companions tugging
on the star.
HD 155358 is slightly hotter than the Sun, but a bit less
massive. Most important, it only contains 20 percent as much
of the chemical elements called "metals" - elements heavier
than hydrogen or helium - as the Sun. Along with one other
star (called HD 47536), it contains the fewest metals of any
star found to harbor planets.
Bean specializes in studying the metal contents of stars.
His in-depth studies of the star's spectrum revealed its
metal-poor nature, and allowed him to deduce the star's age
of roughly 10 billion years.
One planet has an orbital period of 195 days and, at a
minimum, is 90 percent as massive as Jupiter. It orbits HD
155358 at a distance of 0.6 AU. (An astronomical unit, or
AU, is the Earth-Sun distance of 150 million km, or 93
million miles.) The other planet orbits HD 155358 in 530
days, with a minimum mass half that of Jupiter, at a
distance of 1.2 AU.
Wittenmyer used the University of Texas at Austin
supercomputer "Lonestar" to calculate the two massive
planets' orbits 100 million years into the future. The
planets' orbits are not circular, and they orbit close to
each other and thus interact gravitationally -- they push
each other around.
"It's like a dance," Endl said. He explained that "Rob's
calculations show us how the orbits change over time: first
more eccentric, then more circular, and back again." The
system is stable, Endl said, and the pattern repeats about
every 3,000 years.
According to Wittenmyer, "The planets are trading
eccentricity with each other. When one orbit is more
circular, the other is more eccentric."
The combination of massive planets orbiting a metal-poor
star has consequences for theories of planet formation.
"There are two competing planet-formation models," Endl
said. Those models are known as the "core accretion model"
and the "disk instability model."
Both models start with a rotating cloud with a star forming
at its center. As it rotates, the cloud flattens into a
disk. Over time, dust in the disk begins to clump together
to form the seeds that will eventually become planets. Where
the two models differ is in terms of timescale.
In the core accretion model, a Jupiter-like planet forms in
a two-step process. Over about a million years, a
proto-planetary "core" several times the mass of Earth forms
through gravitational accumulation of solid materials. When
it reaches this mass, it has enough gravity to then pull
huge amounts of gas onto itself. Over several million more
years, it grows into a gas giant planet.
This model relies on large amounts of heavy elements to be
present in the disk - and, of course, in the star- to form
the cores, Endl said.
"Most of the planets found using the radial velocity
technique are found around metal-rich stars," he said. "That
argues for the 'core accretion' model. Many astronomers in
this field agree that the higher fraction of planets around
metal-rich stars is supporting evidence for the
core-accretion model."
"Having this process happen to form not just one, but two,
planets around a star that had so little solid material
available for planet-building is quite remarkable." Cochran
said.
The competing model of planet formation is called the disk
instability model. It argues that the rotating disk of gas
and dust around the forming star becomes unstable very soon
after the disk forms, causes the disk to break into giant
clumps. Gravity within each clump can cause the gas to
collapse under its own gravity, forming giant planets in
only several hundred years.
"Gas giant planets formed this way might not have any solid
core at all," Endl said.
Cochran and his colleagues argue that HD 155358 could have
formed the two planets through either method of planet
formation.
"The major result of our discovery is that these planets
required a very massive disk to form, several times more
massive than we think our solar system disk was," Endl said.
"This demonstrates that disk masses can vary significantly
and might even be the most crucial factor in planet
formation."
Cochran and colleagues first began using radial velocity
techniques to search for planets from McDonald Observatory
in the late 1980s, using the 2.7-meter Harlan J. Smith
Telescope. The program continues today on both the Smith
Telescope and HET, and Cochran's team has found planets
orbiting several stars.
The Hobby-Eberly Telescope is a joint project of The
University of Texas at Austin, The Pennsylvania State
University, Stanford University,
Ludwig-Maximilians-Universitat M=FCnchen and
Georg-August-Unversitat Gottingen.
- END -
NOTE TO EDITORS: For more information, please see:
http://austral.as.utexas.edu/planets/hd155358/hd155358.html
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