http://www.abc.net.au/science/news/stories/s1223881.htm
Planets form in a messy muddle
Discovery News
Wednesday, 20 October 2004
Young star with debris disc. The disc formed when embryonic planets
smashed into each other (Image: NASA/JPL-Caltech/T Pyle, SSC-Caltech)
The business of building planets is messier and more chaotic than
scientists realised, according to new observations from NASA's Spitzer
Space Telescope.
Rocky bodies as big as mountain ranges colliding again and again build
planets, a process that probably formed Earth's moon, the telescope
revealed as it focused on large dust clouds around several stars.
The clouds most likely billowed out when nascent, rocky planets
crashed together.
"Our moon took a lot of violent hits when planets had already begun to
take shape," said Professor George Rieke of the University of Arizona,
Tucson, lead author of the findings and a Spitzer scientist.
The findings will be published in the Astrophysical Journal.
"It's a mess out there," Rieke said. "We are seeing that planets have
a long, rocky road to go down before they become full grown."
Infrared vision allowed Spitzer to see the dust generated by the
smashing of mountainous rocks, since the dust glows in the infrared
when it is warmed by the star it surrounds.
Before Spitzer's results, astronomers thought planets were formed
under smoother, calmer circumstances. Conventional theory held that
rocky planets start out as tiny balls in a disc-shaped field of thick
dust.
Over millions of years, the dust particles stick to the balls,
gradually adding to the mass until mountain-sized bodies form and
collide with each other to make planets.
In the study, Spitzer zeroed in on 266 nearby stars of similar size,
two to three times the mass of the Sun and of various ages.
When two worlds collide (Image: NASA/JPL-Caltech/T Pyle, SSC-Caltech)
The researchers expected to see dust discs around the younger stars,
presumably areas of planet formation, and not around the older stars.
Instead, they saw thick dusty circles around some older stars, and
none around some younger suns.
"We thought young stars, about one million years old, would have
larger, brighter discs, and older stars from 10 to 100 million years
old would have fainter ones," Rieke said. "But we found some young
stars missing discs and some old stars with massive discs."
The findings showed that planet-forming discs can be extremely dusty
throughout their lifetimes of hundreds of millions of years, Reike
said.
"The only way to produce as much dust as we are seeing in these older
stars is through huge collisions," Rieke said.
Spitzer's infrared eyes allow it to see the dim infrared light from
discs of different ages.
"These exciting new findings give us new insights into the process of
planetary formation, a process that led to the birth of planet Earth
and to life," said Dr Anne Kinney, director of the universe division
in the Science Mission Directorate at NASA headquarters in Washington,
DC.
--
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