Comets do not deserve a reputation as pure samples of material from the birth of the solar system, because they have been altered in deep space, suggests an analysis published Thursday.

Heat from very bright stars and star explosions called supernovae has altered their characteristics, researchers from the University of Colorado at Boulder suggest."There's no such thing as a pristine comet," said J. Michael Shull, who presented the analysis with colleague S. Alan Stern in Thursday's issue of the British journal Nature.

Comets are essentially dirty snowballs that formed at the birth of the solar system. Scientists study them for clues to the conditions surrounding the formation of the sun and planets.

Trillions of comets are thought to orbit the sun some 10,000 to 100,000 times as far away as the Earth's orbit, Shull said. Those comets have never been seen, but astronomers do spot comets that have left that orbit and swung in toward the sun.

While still in their distant orbits, the analysis contends, the comets have been heated to differing extents by passing stars and by supernova explosions. That caused boiling that obliterated the original structure and chemical composition of the comet surfaces, the analysis says.

Paul Feldman, professor of physics and astronomy at Johns Hopkins University in Baltimore, called the proposal intriguing.

Comets that return to view on a regular schedule have certainly been heated and modified by repeated trips near the sun, he said, but comets making their first trip to the inner solar system are thought to contain pristine material.

"Their hypothesis is not likely to be easily tested by remote observations from the ground or from earth-orbiting observatories but will await future missions to comets which can actually measure the composition and structure of the ice directly" he said.