Nearby Star System Could Spawn Carbon-Rich Planets

John Roach
for National Geographic News
June 7, 2006
An unexpectedly large amount of carbon—the basis of life as we know it—surrounds a young nearby star where astronomers believe rocky planets are forming, according to a new study.

Such an environment might spawn tar-covered, diamond-rich worlds and life-forms that consume oxygen-rich foods, one scientist speculates.

The carbon discovery challenges ideas about the planets forming around the star in question, Beta Pictoris. The star is 63 light-years away and about twice the size of our sun.

"This is different than we expected," said Aki Roberge, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

Many astronomers believe the Beta Pictoris system is similar to our solar system when it was much younger and the planets, including Earth, were forming. Beta Pictoris is thought to be between 8 and 20 million years old. Our solar system is at least 4.54 billion years old, scientists say.

Some evidence suggests a giant gas planet like Jupiter has already formed in the dusty disk around Beta Pictoris. Scientists believe terrestrial planets—planets with land, such as Mercury, Venus, Earth, and Mars—are forming right now.

(National Geographic magazine photos: "Search for Other Earths.")

Unexplained Carbon

Until now it's been a mystery as to why Beta Pictoris is enveloped in a gaseous disk, Roberge says.

Astronomers believed the gas was primarily composed of metal elements such as iron and silicon, which should have been getting blown away by radiation streaming out from the star.

"We shouldn't see [the clouds] where they are," Roberge said. "But we see them calmly in the disk orbiting the star. All previous theoretical work couldn't solve this."

Astronomers came to believe a hidden mass of gas, perhaps hydrogen, slowed the clouds' outflow.

Roberge and her colleagues used NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and the Hubble Space Telescope to measure the gases. They found an unexpectedly high quantity of carbon gas—about 20 times more than in our own solar system.

Carbon "doesn't get blown away like other gases do, and if there's enough, it can bind the other gases to the star," Roberge said.

"Our measurements show more than enough [carbon] for that to work—to keep them in orbit instead of being blown away."

She and her colleagues report the finding in tomorrow's issue of the journal Nature.

Carbon Source

The finding, however, does not explain what put the carbon there in the first place. Roberge says scientists have several ideas.

Conel Alexander is a cosmochemist in the Department of Terrestrial Magnetism at the Carnegie Institution of Washington, D.C. He explored two of the possibilities in a telephone call with reporters.

"Perhaps because Beta Pictoris is such a young system, what we're seeing is a snapshot of what occurred in [our] early solar system as the asteroids were degassing, while the [life-forms] in them were being cooked in their interiors," he said.

Alternatively, he said, the carbon may be leftover gas that failed to condense after the energetic collisions of planet-forming asteroids and comets.

But neither explanation, he adds, is explained by the current observations.

"We need to at least consider a more exotic explanation," he said.

Marc Kuchner studies planet formation at the Goddard Space Flight Center. He suggests that the carbon-rich planets forming around Beta Pictoris are unlike those in our solar system.

The Beta Pictoris carbon planets would have a core composed of iron, nickel, and carbon, he says. Closer to the surface, but still deep underground, might be a layer of graphite.

"Of course graphite wouldn't exist deep underground," he said. "Like coal, graphite would change into diamond at high pressures."

The surface of such a planet would be covered in tar, soot, and other carbon-rich materials.

If any life existed on such a planet, it would survive by consuming oxygen-rich foods and using carbon to derive energy from them—the reverse of what we do on Earth.

"Maybe Beta Pictoris is a glimpse at the history of our own solar system," he said.

"Or maybe instead Beta Pictoris is telling us about the startling variety of other kinds of planetary systems that might be out there."

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