Image: TW Hydrae
Bill Saxton  /  NRAO / AUI / NSF
The TW Hydrae system, shown in this artist's conception, contains a protoplanetary disk holding vast numbers of pebble-sized rocky chunks that should eventually grow into full-sized planets.
By Senior science writer
updated 6/24/2005 5:47:57 PM ET 2005-06-24T21:47:57

A vast collection of space pebbles surrounding a relatively nearby star is a planetary construction zone, astronomers say.

The star, TW Hydrae, is young and ripe for developing new worlds.

New observations reveal a swath of pebble-sized material extending at least a billion miles from the star. It's just the sort of stuff theory says is needed for making comets, asteroids and eventually planets around a young star.

"We're seeing planet building happening right before our eyes," David Wilner of the Harvard-Smithsonian Center for Astrophysics said Friday.  "The foundation has been laid, and now the building materials are coming together to make a new solar system."

Right age
After a star is born, it takes millions of years for a planet to form, astronomers believe. TW Hydrae is about 10 million years old.

Only a few sets of observations have shown the planet-formation process in progress, and scientists have yet to witness all the phases, thereby letting them piece together a full chain of events based not just on theory but real evidence. It's also not known if there is just one primary mechanism for building a planet or if there might be two or more.

Wilner and colleagues used the National Science Foundation's Very Large Array to measure radio emissions from TW Hydrae. The length of the radio waves suggests the size of particles from which they emanate.

"The strong emission at wavelengths of a few centimeters is convincing evidence that particles of about the same size are present," said co-researcher Mark Claussen of the National Radio Astronomy Observatory. "No one has seen this before."

One project completed?
One giant planet may already have formed.

A computer simulation of the disk of material around the star, based on previous infrared observations, reveals a gap that 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, the astronomers say.

Located about 180 light-years away in the constellation Hydra the Water Snake, TW Hydrae is nearly as massive as our sun.

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

This results were detailed June 20 in Astrophysical Journal Letters.

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