Image: HD 69830 asteroid belt
T. Pyle / SSC / NASA / Caltech
An artist's conception shows the haze of a thick asteroid belt around the star HD 69830, with a blue planet in the foreground. Scientists believe such an asteroid belt could hint at the presence of Earthlike planets.
By Senior science writer
updated 4/20/2005 11:15:22 PM ET 2005-04-21T03:15:22

Wherever there might be Earthlike planets, there are likely to be asteroids, too. And now astronomers have found evidence for an asteroid belt around another star similar to our sun.

Previous studies detected asteroid belts around other stars. But the newfound collection of space rocks — if that's what it is — is in a setup that might resemble our solar system, scientists announced Wednesday.

Asteroids in our solar system are said to be the detritus of planet formation. They are the objects that either did not grow large enough to achieve planet status, or they were broken apart by collisions back when the solar system was more chaotic.

Those smash-ups, in fact, are the theoretical key to the new discovery.

Follow the dust
Asteroid collisions create lots of dust. NASA's Spitzer Space Telescope was used to spot a thick swarm of dust around a star called HD 69830, 41 light-years away. The scientists believe the dust represents a belt of asteroids in which major collisions occur every 1,000 years or so.

"Because this belt has more asteroids than ours, collisions are larger and more frequent, which is why Spitzer could detect the belt," said George Rieke of the University of Arizona. "Our present-day solar system is a quieter place, with impacts of the scale that killed the dinosaurs occurring only every 100 million years or so."

The results will be detailed in the Astrophysical Journal.

HD 69830 is not exactly a twin to our solar system.

The local asteroid belt sits between Mars and Jupiter. The HD 69830 belt is inside a region equivalent to the orbit of Earth, or perhaps half the diameter of Earth's orbit — its exact extent is not clear. HD 69830's belt contains about 25 times more material. If there's a planet around the star, its night sky would be graced by a band of light, similar to but 1,000 times brighter than the zodiacal light seen from Earth.

"The night sky would be quite spectacular," said lead researcher Charles Beichman of the California Institute of Technology.

A shepherd planet?
There are similarities, too. Here, Jupiter's immense gravity shepherds the outer edge of the asteroid belt. An unseen planet perhaps the size of Saturn or even smaller might play that role around HD 69830, the researchers say.

Present technology is incapable of detecting Earth-sized planets around sunlike stars, which makes this new study compelling to planet hunters.

Interactive: The search for extrasolar planets "Asteroids are the leftover building blocks of rocky planets like Earth," Beichman said. "We can't directly see other terrestrial planets, but now we can study their dusty fossils."

Observations do not reveal whether there are any planets around HD 69830. In a NASA-run teleconference with reporters, researchers said a rocky planet cold be possible. Because the star is only about half as powerful as the sun, however, its "habitable zone" would be confined to a region closer to the star than Earth is to the sun.

If the asteroid belt around HD 69830 extends to the equivalent of Earth's orbital diameter, then there would be no room left for a habitable planet, said Jonathan Lunine, a planetary scientist at the University of Arizona. If the asteroid belt is confined to half an Earth-orbit diameter, then a rocky planet could have formed and might be close enough to the star to harbor liquid water, said Lunine, who was not involved in the study.

There is an outside chance the presumed asteroid belt is instead dust boiling off a comet the size of Pluto.

"The 'supercomet' theory is a more of a long shot," Beichman said. More observations by Spitzer and ground-based telescopes are planned. "We'll know soon enough."

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