Asteroid Belt Discovered Around Our Sun's "Twin"

Brian Handwerk
for National Geographic News
April 21, 2005
NASA's orbiting Spitzer Space Telescope has found evidence of a massive asteroid belt around a "twin" of our own sun.

Kim Weaver, a Spitzer Space Telescope scientist, said the finding marks "the first time that scientists have found evidence for a massive asteroid belt around a mature, sunlike star."

"This region around the star is the sort of place where rocky planets [like Earth] may form," Weaver said yesterday at a press conference from NASA headquarters in Washington, D.C.

The star, dubbed HD69830, is some 41 light-years away—which, in space terms, is practically our own backyard. Part of the constellation Puppis, the star is a tad too faint to see with the unaided eye.

The discovery may help reveal how other Earth-like planets could be formed and whether our own solar system is common or unique in space.

Construction Site or Junkyard?

"Asteroids are the leftover building blocks of rocky planets like Earth," said Charles Beichman, a Spitzer Space Telescope scientist based at the California Institute of Technology in Pasadena.

Beichman is the lead author of a paper describing the new asteroid belt, which will be published in Astrophysical Journal.

"We're interested in asteroid belts in these systems," Beichman said, "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 places where, for one reason or another, material just couldn't assemble to form planets at all. "

Asteroids occassionally collide with each other, raising cosmic dust. They also crash into planets and moons.

The new asteroid belt was signalled by a thick disk of small dust grains that star HD69830 warms to temperatures that range from room temperature to 450° Fahreinheit (232° Celsius).

Of the 85 stars Spitzer scientists have examined to date, only HD69830 yielded evidence of an asteroid belt. It is thicker than the asteroid belt in our own solar system, which lies between Mars and Jupiter and packs nearly 25 times more debris.

George Rieke is a study co-author and principal investigator with the Spitzer Space Telescope who is based at the University of Arizona in Tucson. "Because this belt has more asteroids than ours, collisions are larger and more frequent, which is why Spitzer could detect the belt," he said.

"Our present-day solar system is a quieter place, with impacts of the scale that killed the dinosaurs occurring only every hundred million years or so," he added.

Astronomers have previously detected other asteroid belts around two far younger, more massive suns. But researchers believe the latest discovery—an asteroid belt arrayed around a mature star—will reveal more about our sun and whether our solar system is the norm or the exception.

Earth-Like Planets

"We all want to understand, ultimately, how common our own solar system is and how common habitable planets like our own Earth might be throughout the cosmos," said Jonathan I. Lunine, a physics and planetary science professor at the University of Arizona.

Scientists say they don't know yet if any planets orbit HD69830. But they do know where an Earth-like planet—featuring liquid water on its surface for billions of years—would have to be located.

HD69830 "is a bit dimmer and younger than our own sun, perhaps half the brightness of our own sun," Lunine explained. "So to have an 'Earth' with the same conditions we'd want it to move from 1 astronomical unit [AU] to 0.9 or perhaps 0.8 AU from that star," he said.

One AU represents the mean distance from the Earth to the sun, about 93 million miles (150 million kilometers).

It may or may not be possible for a planet to exist in what some dub the Goldilocks zone (not too hot, not too cold).

Knowing how closely the new asteroid belt orbits its star can tell scientists whether or not an Earth-like planet is possible in this belt, Lunine said.

Even if a terrestrial planet does exist in the Goldilocks zone it would be peppered with asteroid impacts. Extinction-causing impacts on such a planet would likely occur about once every million years—making it debatable whether life could have ever taken hold.

Giant Comet?

Scientists have yet to definitively determine that HD69830 does, indeed, have a massive asteroid belt. They must first rule out a second statistically improbable but intriguing explantion for the Spitzer discovery.

It is possible that the dust detected by Spitzer is from a giant, Pluto-size comet that was bounced into the inner solar system via a "cosmic bank shot" and is leaving a trail of dust as it distinigrates.

The dust does contain silicate crystals, like fosterite, similar to those found in the famous comet Hale-Bopp.

But the theory is a statistical longshot. It is not nearly as likely as the asteroid belt concept, Spitzer scientists say.

Spitzer and ground-based telescopes may soon confirm the asteroid belt theory by scanning the region for water and carbon monoxide—compounds common in comets, but not asteroids.

After confirming that their new observation is indeed an asteroid belt, scientists could focus their attention on the building blocks of a distant star system much like our own.

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