Science / Medicine : A Weekly Roundup of News, Features and Commentary : Researchers Take New Look at Comets

Comets may be less like the material that existed in the birth of the solar system than most astronomers believe, University of Colorado at Boulder researchers speculate. Instead, they say, comets’ characteristics have been altered over the last 4.5 billion years by heating from unusually bright stars and star explosions called supernovae.

“There’s no such thing as a pristine comet,” said J. Michael Shull, who presented the analysis with colleague S. Alan Stern in the current issue of Nature.

Scientists study comets, which are essentially dirty snowballs that formed at the birth of the solar system, 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, Shull and Stern contend, the comets have been heated to differing extents by very bright stars and by supernova explosions. That causes boiling that obliterates the original structure and chemical composition of the comet surfaces, they say.