An orbiting U.S. telescope has spotted what could be the dusty remnants of asteroids colliding near a star very much like our Sun. Asteroids are the leftover building blocks of rocky planets like Earth. If the finding is what astronomers believe it is, they say it may be a significant step toward learning if and where other Earths form.

The U.S. space agency's Spitzer Space Telescope has detected a thick disk of warm dust around a star similar in size and age to our sun 40 light years away. Astronomers think it could be from a massive asteroid belt 25 times bigger than the one in our solar system.

Spitzer did not see actual asteroids near the star, but the scientists who made the observation suggest that frequent asteroid collisions are the most-likely source of the dust Spitzer did see. They believe such collisions occur about every one thousand years, far more often than in our solar neighborhood.

Co-discoverer Charles Beichman of the Jet Propulsion Laboratory in California says this is the first asteroid belt ever seen around a mature star like the Sun.

"We're interested in asteroid belts in these systems because they may mark either the construction sites that accompany the formation of rocky planets, the junkyards that remain after the formation of such planets, or simply mark places for one reason or another material just couldn't assemble to form planets at all,? he said.  ?But in particular, we're really interested in understanding more about the asteroid belts of mature stars because they tell us more about our own Sun and whether our own planetary system is the norm or exceptional."

The scientists say if there were a habitable Earth-like planet around the star, the dense layers of dust created by the frequent asteroid collisions would light up its night sky as a brilliant band.

But they say that whether such a habitable planet exists depends on how wide the presumed asteroid belt is. If the belt is narrow and does not extend out to the habitable zone where mild temperatures allow water to be liquid and life to flourish, a planet in this region could sustain life. However, the high density of the asteroid belt would lead to frequent collisions with the planet and cause mass extinctions.

If the asteroid belt is wider and extends beyond the habitable zone, its very density would be a sign that no planets formed within it.

Co-researcher Jonathan Lunine of the University of Arizona says that if a planet were beyond the asteroid belt and out of the habitable zone, it might be too cold to harbor life as we know it.

"So knowing exactly where the outer edge of that belt is will be crucial in understanding whether there is a habitable planet located in that system," he added.

The astronomers concede that what Spitzer observed might not be the dusty remains of asteroid collisions after all, but a giant comet, releasing dust from its ice as the ice boils away in its orbit around the star. This thought occurred to them when they discovered that the dust around the star consists of small silicate crystals like those seen in the comet Hale-Bopp.

Charles Beichman says future observations might provide the answer.

"We have a number of observations planned in the coming year where we will try to look at the spectrum in slightly greater detail,? he explained.  ?There is also some ground-based telescope work that we're investigating that might let us look for the gas and the water that might be associated with a comet if that were the source of this dust. Those observations will help us rule in or out the comet hypothesis."

If an asteroid belt around this star is confirmed, the scientists say more powerful U.S. orbiting telescopes now being planned could hunt for planets in the system. One mission is the Terrestrial Planet Finder to be launched at the end of the next decade to search specifically for Earth-like planets around other stars.