Astronomers studying a disk
of material circling a still-forming star inside our
galaxy have found a tantalizing result—the
inner part of the disk is orbiting the protostar in
the opposite direction from the outer part of the disk.
“The solar system that likely will be formed
around this star will include planets orbiting in different
directions, unlike our own solar system in which all
the planets orbit the Sun in the same direction,” said
CISTO senior scientist J. Michael Hollis.
“This is the first time anyone has seen anything
like this, and it means that the process of forming
planets from such disks is more complex than we previously
expected,” said Anthony Remijan, a former CISTO
post-doctoral researcher now at the National Radio
Astronomy Observatory. Remijan and Hollis used the
National Science Foundation's Very Large Array (VLA)
radio telescope to make the discovery.
Stars and planets, scientists believe, are formed
when giant clouds of gas and dust collapse. As the
cloud collapses, a flattened, rotating disk of material
develops around the young star. This disk provides
the material from which planets form. The disk and
the resulting planets rotate in the same direction
as the original cloud, with the rotation speed increasing
closer to the center, much as figure skaters spin faster
when they draw their arms inward.
If all the material in the star and disk come from
the same prestellar cloud, they all will rotate in
the same direction. That is the case with our own solar
system, in which the planets all orbit the Sun in the
same direction as the Sun itself rotates on its axis.
In the case of a young star some 500 light-years from
Earth in the direction of the constellation Ophiuchus,
Remijan and Hollis found the inner and outer parts
of the disk rotating in opposite directions.
“We
think this system may have gotten material from two
clouds instead of one, and the two were rotating in
opposite directions,” Remijan said. There is
sufficient material to form planets from both parts
of the disk, he added. The object is in a large, star-forming
region where chaotic motions and eddies in the gas
and dust result in smaller cloudlets that can rotate
in different directions.
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Top
view: A
huge star-forming region is rotating globally in the
direction shown by the white arrow. This large region
can give birth to multiple stellar systems. Middle
view: A detailed view inside the large star-forming
region shows three protostars forming as the region
collapses. The collapse process is chaotic and can
cause eddies, allowing newly-forming stars to rotate
in different directions and at different speeds, as
shown by the arrows. Bottom view: One protostellar
cloud collapses further into a disk-like structure
that rotates counter-clockwise (white arrows) about
the newly-formed protostar. In addition, the protostar
siphons off material from a second, passing protostellar
cloud rotating in the opposite direction. Because
of this process, the outer part of the disk rotates
clockwise (yellow arrows). Eventually, planets
will form from the material in this disk, with
the outer planets orbiting the star in the opposite
direction from the inner planets (Image credit:
Bill Saxton, NRAO/AUI/NSF).
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In the solar system that
probably will form around this young star, the innermost
planets will orbit in one direction and the outer planets
will orbit in the opposite direction.
The scientists studied the star-forming clouds by
analyzing radio waves emitted at specific, known
frequencies by molecules within the clouds. Because
the molecules emit radio waves at specific frequencies,
shifts in those frequencies caused by motions (called
Doppler Shift) can be measured, revealing the direction
in which the gas is moving relative to Earth.
The newest VLA observations of the region showed the
motion of silicon monoxide molecules, which emit radio
waves at about 43 GigaHertz. When the astronomers compared
their new VLA measurements of the motion of silicon
monoxide molecules close to the young star with earlier
measurements of other molecules farther away from the
protostar, they realized the two were orbiting the
star in opposite directions.
Although this is the first time such a phenomenon
has been seen in a disk around a young star, “Similar
structures and dynamics commonly occur on small and
large scales throughout the universe,” said Hollis. “Thus,
it is not surprising to find counter-rotation in a
protostellar disk since the phenomenon has been previously
reported in the disks of galaxies.”
The
astronomers’ results appeared in the April 1
edition of the Astrophysical Journal.
The National Radio Astronomy Observatory is a National
Science Foundation facility operated under a cooperative
agreement by Associated Universities, Inc.
http://www.nasa.gov/centers/goddard/news/topstory/2006/opposite_orbit.html
http://www.nrao.edu/pr/2006/counterdisk
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