Comet Hale-Bopp, which paraded across evening skies during the winter and early spring of 1997, may have formed in our solar system somewhere near Neptune, a team of astronomers announced today. If correct, this would be the first time scientists have been able to pin down the homeland of one of the solar systems icy wayfarers.

The conclusion was based on the first-ever measurement of a noble gas in a comet, which revealed it has an abundance of argon, said Alan Stern, leader of a team of researchers that announced the finding today at the American Astronomical Society's meeting in Rochester, New York. Stern directs the Southwest Research Institute's Space Studies Department, and is principal investigator of the team that analyzed Hale-Bopp.

Cometary scientists have yearned for information about the amount of noble gasses in the icy tail draggers because it can serve as a record of the temperatures that the comets have experienced during their lifetimes.

The farther away from the sun, the colder the temperature, so knowing the thermal history of an object can help scientists determine where in the solar system an object formed and developed.

The noble gasses, such as helium, neon, argon, krypton, xenon and radon, are inert -- they dont react with other elements and they exist as ice only at extremely low temperatures. Because of this, the discovery of a certain noble gas tells scientists that an object has spent most its life without heating above the temperature where that gas would steam off into space.

In space, argon is solid only below about 35 degrees Kelvin (minus 395 degrees Fahrenheit, or minus 238 degrees Celsius).

The presence of abundant argon means that the comet has formed in a region of space that is no hotter than 35 degrees Kelvin, Stern said. Some scientists have guessed that Hale-Bopp originated near Jupiter, but the argon finding rules out this possibility because that region is likely too warm for argon.

Another piece of the puzzle was filled in by information from a NASA satellite which searched for neon a noble gas that has a melting point below that of argon. It detected no neon in Hale-Bopp, a fact that means the comet formed above about 20 degrees Kelvin (minus 424 degrees Fahrenheit or minus 253 degrees Celsius), Stern said.

This profile, where the general temperature is above 20 degrees but below 35 degrees Kelvin, matches the temperature of the Neptune region, an area some 30 times farther from the sun than Earth.

"The combination of these two results," Stern said, "tells us that this comet has been heated through and through between about 20, but no more than about 35 degrees Kelvin. Those two temperatures trap, or bound the maximum temperature this temperature has ever been heated to through and through. And that corresponds to telling us that the comet can't have been born in the Jupiter region of the solar system..., but must have formed further out where it was cool enough that it never saw a temperature like 35 Kelvin. In fact it indicates that it was probably born somewhere in the Uranus-Neptune [region] or, possibly, even further out."

Scientists aren't certain where comets come from, but until now, most scientists were fairly comfortable with the prediction that Hale-Bopp was born near Jupiter.

But Stern's work strongly supports the hypothesis that the Neptune region is the source of some of them, said Lucy McFadden, a comet expert at the University of Maryland, College Park. McFadden, who is not involved in Stern's research, called the argon finding a "spectacular accomplishment."

"If its got these temperatures indicating that it came from the Neptune region, that just tells us that the material from Hale-Bopp is material that was there from the beginning from the Neptune region, rather than the deep freeze out in the Oort cloud, which would have a different composition and be contaminated by interstellar materials," McFadden said.

Thus, Hale-Bopp may be a time capsule that holds all kinds of answers about how Neptune formed, and the early history of the solar system out in the Neptune-Uranus region.

"This really whets the appetite for further observations," Stern said in an interview. "We now have one data point, but there are a trillion comets in the Oort cloud. This opens the field of comparative study."