Shock Zone Found Where Planets are Born

In research that may help reveal how planets form,
astronomers have gathered evidence that a shock is created
when material falls in toward a dust disk around a growing
star.

“This is a very important step in our understanding of
how stars and planets develop,” said Dr. Thangasamy Velusamy,
principal research scientist at NASA’s Jet Propulsion
Laboratory, Pasadena, Calif. “It gives us some insight into
the process by which planetary systems could begin around
stars.” Velusamy and his colleagues will present their
findings today at the American Astronomical Society winter
meeting in Washington, D.C.

Dusty disks around stars, called protostellar disks, form
from gas and dust in interstellar clouds. The disks grow as
they accumulate material falling in from the parent cloud.
Where the cloud material meets the disk, astronomers had
predicted that a warm shock would occur. This new discovery
confirms that prediction.

Scientists believe dust particles in these disks clump
together, eventually leading to small rocks, which can join
together to form planets and comets. Scientists believe that
a similar process formed Earth and the other planets of our
solar system from a dusty disk around our parent star, the
Sun.

At the shock zone, the newly introduced material slows
down and redistributes itself throughout the disk. Some will
make its way toward the center and become part of the
developing star, while some will eventually become part of the
planets.

Protostellar disks, like the one observed by Velusamy and
his colleagues in cloud Lynds 1157 (located about 1,300 light
years from Earth in the constellation Cepheus), contain mostly
molecular hydrogen. Other ingredients include tiny dust
particles and trace amounts of such chemicals as carbon
monoxide and methanol, or rubbing alcohol. Because the
falling material and the disk are so cold, these chemicals
freeze on the dust particles and can not be seen. However,
when a shock is produced at the disk’s surface, the resulting
heat warms the dust particles and releases their icy mantles.
This transforms the ice into a gas that emits radiation in
radio wavelengths, which can be detected.

Using the Owens Valley Radio Observatory Millimeter Array
near Bishop, Calif., the team of scientists, including Dr.
William Langer, senior research scientist at JPL, and Dr. Paul
Goldsmith, a professor of astronomy at Cornell University,
Ithaca, N.Y., confirmed the presence of a dusty disk in L1157.
They also found the first-ever evidence for the shock region.
It was detected when telltale disk emissions indicated the
presence of methanol only at the predicted shock zone.

“The methanol emission looks different than the
distribution of material already in the disk,” noted
Goldsmith.

“Methanol is intriguing, since it is abundant in both
interstellar space and in comets, and is a chemical starting
point for more complex molecules, including those that
eventually form life,” said Langer.

Protostellar disks are planetary construction zones.
Over the past several years, astronomers have discovered
planets orbiting dozens of other stars. The key mystery
waiting to be solved is whether any extrasolar planet may have
conditions suitable for life, and if so, whether life exists
there.

The latest findings will appear in the January 20, 2002
issue of the Astrophysical Journal Letters. The scientists
plan further research on the chemical makeup of planet-forming
disks. Future NASA missions will also study these dust disks.
The Space Infrared Telescope Facility may study ice crystals
in the disks as part of its investigations. Other planned
missions, including the Terrestrial Planet Finder, will be
able to image planetary systems forming in disks around other
stars. Both missions are managed by JPL.

An image of the new observation is available online at
http://www.jpl.nasa.gov/images/sg_other . The Owens Valley
Radio Observatory Millimeter Array, operated by the California
Institute of Technology in Pasadena, is supported in part by
the National Science Foundation. Caltech manages JPL for
NASA.