Microscopic space dust from the comet Wild 2 reveals clues that seem to run counter to the basic theories of the solar system's origin, U.S. scientists say.

Samples of dust, gathered from the Stardust mission, indicate the comet formed in the Kuiper Belt, outside Neptune's orbit, and only recently entered the inner areas of solar system, Lawrence Livermore National Laboratory scientists said in California. During its lifetime the comet gathered material that formed much closer to the sun.

"We're talking about a mineral that forms around 3,000 degrees Kelvin, which means it formed close to the hot infant star," said John Bradley, director of the lab's Institute for Geophysics and Planetary Physics and the head of the Livermore Stardust team. "If we found it in the comet, then how the heck did it get out there."

The mineral is osbornite, which has been found in Russia. Finding osbornite in the comet indicates that the solar system was more volatile during its infancy than originally thought, Bradley said.

The comet samples also had pre-solar materials that were transported beyond the orbit of Neptune by some process capable of moving particles, Bradley said.

The multinational Stardust findings were published in the journal Science.