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Birth of an Earthlike Planet Spied By Spitzer

Mason Inman
for National Geographic News
October 3, 2007
 
A warm belt of dust around a young star is offering astronomers a glimpse of what Earth might have looked like when it was just beginning to take shape.

The star is part of a binary system known as HD 113766 that lies 424 light-years away.

Although it is slightly more massive than our sun, the star is only about 16 million years old—a baby compared to our 4.6-billion-year-old solar system.

And this young star seems to be in the early stages of forming its own rocky planets, a new study suggests.

The star system HD 113766 stands out clearly when looked at in infrared light, the part of the spectrum where dust shows up best.

Based on infrared data from NASA's Spitzer Space Telescope, astronomers say that the dust seems to be collected in a ring around the star within its "habitable zone," the region where water can stay liquid (see images from Spitzer).

Although the researchers can't see if any larger rocks have taken shape inside the dust, the abundance of material suggests that there's enough to form at least a Mars-size planet—or perhaps an Earth-size one.

"You've got all the right kinds of stuff—the age, the mass, the right location," said Carey Lisse of Johns Hopkins University in Maryland.

Lisse and colleagues will present their findings in an upcoming issue of the Astrophysical Journal.

Dust in Infrared

By examining the composition of the warm dust disk, Lisse's team was able to pinpoint what sort of body is most likely forming around the star.

A special instrument aboard Spitzer can search in the infrared spectrum for the fingerprints of particular molecules in the distant dust.

"We could actually tell what kind of dust is there" in the HD 113766 system, Lisse said. "In our case, it's rock and metal."

For example, the dust is rich in iron sulfides, a group of metals that includes the mineral pyrite, or fool's gold.

Lisse's team also noted what was not present in the dust: water ice and complex but fragile molecules known as hydrocarbons.

Thanks to the Deep Impact mission that analyzed the composition of Comet Tempel 1, the scientists knew that such materials would be expected if the dust was giving birth to comets.

(Read "Frozen Water Discovered on 'Deep Impact' Comet" [February 2, 2006].)

And studies of meteorites that have made their way to Earth suggest that dust coming off of large asteroids should not have as much metal.

This means that the astronomers are seeing HD 113766 just as the predecessor of a rocky planet, known as a planetesimal, is forming.

What's more, a cooler disk farther out from the star appears to contain icy dust, a possible source of water for the newborn planet.

For now, the proto-planet is probably zooming around inside the dust disk, Lisse said. And we may be able to catch signs of the system as its planet grows up.

HD 113776 has been observed a few times, "and we know [the amount of dust] hasn't died down too much over 20 years," Lisse said.

"If it's at the end [of forming planets], there would be a lot of bumping and scraping, but we might actually see it dying down" in the near future.

"Or if it's young, it could flare up," showing occasional signs of more collisions between rocks, which would stir up more dust.

"Stay tuned," Lisse added. "I expect lots more fireworks as the planet in HD113766 grows."

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