Hurtling through space in the farthest reaches of our solar system, icy comets are one of the last extraterrestrial bodies that scientists expected to contain matter formed at the sun. But new findings based on samples from NASA's Stardust mission—winner of a 2006 Popular Mechanics Breakthrough Award for bringing back the oldest material ever collected upon its fiery return to Earth—indicate that comets are made up of more diverse materials than previously understood.

"The general consensus was that comets formed in isolation on the edge of the solar system," says Donald Brownlee, a professor of astronomy at the University of Washington and lead scientist on the Stardust mission, who spoke with us after Stardust team leader Joe Villenga joined a PM roundtable discussion previewing these findings. "We expected to find particles that formed around other stars."

But some of the samples captured from comet Wild 2 during the Stardust flight proved to be calcium-aluminum—a material that dates back 4.6 billion years and formed in the fiery center of the solar system as the sun was born. "It's like going into your backyard and finding a rock from Siberia," Brownlee says.

The findings, which will be published tomorrow in the journal Science, leave scientists pondering how such matter arrived in the farthest reaches of the solar system in the first place. Brownlee points to two theories: One posits that a ballistic process fired matter from close to the sun into outer space, while the other points to a gentle, swirling eddy that transported the material more slowly.

Further evidence might be needed to validate either theory, but the Stardust samples do offer concrete proof of the outer solar system’s composition as it formed. Brownlee says the next step is to compare these particles to samples from meteorites, which formed between Mars and Jupiter. –Emily Masamitsu