Oct. 19, 2004 — The business of building planets is messier and more chaotic than scientists realized, 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 in 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 George Rieke of the University of Arizona, Tucson, lead author of the findings and a Spitzer scientist, in a press release.

The findings will be published in an upcoming issue of 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, the press release said. 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 various ages. They 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 all 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 Anne Kinney, director of the universe division in the Science Mission Directorate at NASA Headquarters in Washington, D.C.