Prevailing theories about the solar system's formation cannot explain how this high-temperature material ended up in the frigid regions beyond Neptune's orbit, the so-called Kuiper Belt where comets formed.


"Remarkably enough, we have found fire and ice," Stardust principal investigator Donald Brownlee, with the University of Washington, said on Monday.


"We've found samples from the coldest part of the solar system that have mineral grains formed under extremely high temperatures."


The grains include an exotic titanium-vanadium-nitrogen blend that forms in temperatures higher than 1100 degrees Celsius.


Two conclusions


The discovery leads scientists to two possible conclusions: either the minerals formed in the hottest, innermost region of the nebula that eventually became our solar system, or they came from another star.


"If it was formed in our solar system, then it had to be transported from the hottest regions to the coolest," Brownlee said.


Stardust space probe captured
the first samples of comet dust

Scientists estimate the minerals would have had to be closer to the sun than Mercury to form and they would have had to be flung more than 45 times the distance of Earth to the sun to reach the Kuiper Belt region where the comets were born.


Researchers plan extensive studies to determine the chemical histories of the sample particles, which eventually will reveal whether the grains were heated by the sun or another star.


"We will know soon," Brownlee said at the annual Lunar Planetary Institute's science conference, which opened on Monday in Houston.


The Stardust science spacecraft was launched in February 1999 and flew by Comet Wild-2 on 2 January 2004, on a mission to retrieve samples for analysis. A capsule containing the comet samples parachuted to Earth two months ago.


Light on theory


In addition to learning about how the solar system formed, scientists hope the Stardust samples will shed light on a theory that comets supplied primitive Earth with the organic materials needed for life.


Nasa astrophysicist Scott Sandford, with the Ames Research Center in Moffett Field, California, said it is excruciatingly difficult to sort out cosmic organic matter from terrestrial elements, but at least one sample from the Stardust collection seems to contain indigenous carbon and hydrogen.


Chief investigator Brownlee (L)
said 'we have found fire and ice'

Chemical analysis of the sample shows high ratios of deuterium, a hydrogen isotope that is double the mass of ordinary hydrogen due to a neutron in the nucleus.


"We take this as a pretty promising sign, but we need more work," Sandford said.


Between 150 and 200 samples from the comet are currently circulating among scientists and research labs around the world. Next month, work begins on another set of samples collected during the mission, interstellar dust grains.


Unlike the comet samples, the bits of interstellar dust picked up during Stardust's travels are tiny.


Scientists plan an internet-based detection programme of volunteers who will use their home computers to scrutinise images and try to identify any particles.


The project, called Stardust at Home, begins on 2 April.