Detour: Planetary Construction Zone Ahead

http://www.cfa.harvard.edu/press/pr0521.html

Harvard-Smithsonian Center for Astrophysics
Release No.: 05-21
For Release: Friday, June 24, 2005

Note to editors: High-resolution artwork to accompany this release is
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Detour: Planetary Construction Zone Ahead

Cambridge, MA--Interstellar travelers might want to detour around the
star system TW Hydrae to avoid a messy planetary construction site.
Astronomer David Wilner of the Harvard-Smithsonian Center for
Astrophysics (CfA) and his colleagues have discovered that the gaseous
protoplanetary disk surrounding TW Hydrae holds vast swaths of pebbles
extending outward for at least 1 billion miles. These rocky chunks
should continue to grow in size as they collide and stick together
until
they eventually form planets.

"We're seeing planet building happening right before our eyes," said
Wilner. "The foundation has been laid and now the building materials
are
coming together to make a new solar system."

Wilner used the National Science Foundation's Very Large Array to
measure radio emissions from TW Hydrae. He detected radiation from a
cold, extended dust disk suffused with centimeter-sized pebbles. Such
pebbles are a prerequisite for planet formation, created as dust
collects together into larger and larger clumps. Over millions of
years,
those clumps grow into planets.

"We're seeing an important step on the path from interstellar dust
particles to planets," said Mark Claussen (NRAO), a co-author on the
paper announcing the discovery. "No one has seen this before."

A dusty disk like that in TW Hydrae tends to emit radio waves with
wavelengths similar to the size of the particles in the disk. Other
effects can mask this, however. In TW Hydrae, the astronomers
explained,
both the relatively close distance of the system and the stage of the
young star's evolution are just right to allow the relationship of
particle size and wavelength to prevail. The scientists observed the
young star's disk with the VLA at several centimeter-range wavelengths.
"The strong emission at wavelengths of a few centimeters is convincing
evidence that particles of about the same size are present," Claussen
said.

Not only does TW Hydrae show evidence of ongoing planet formation, it
also shows signs that at least one giant planet may have formed
already.
Wilner's colleague, Nuria Calvet (CfA), has created a computer
simulation of the disk around TW Hydrae using previously published
infrared observations. She showed that a gap extends from the star out
to a distance of about 400 million miles - similar to the distance to
the asteroid belt in our solar system. The gap likely formed when a
giant planet sucked up all the nearby material, leaving a hole in the
middle of the disk.

Located about 180 light-years away in the constellation Hydra the Water
Snake, TW Hydrae consists of a 10 million-year-old star about
four-fifths as massive as the Sun. The protoplanetary disk surrounding
TW Hydrae contains about one-tenth as much material as the Sun - more
than enough to form one or more Jupiter-sized worlds.

"TW Hydrae is unique," said Wilner. "It's nearby, and it's just the
right age to be forming planets. We'll be studying it for decades to
come."

This research was published in the June 20, 2005, issue of The
Astrophysical Journal Letters.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

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