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Carbon excess enshrouds star's solar system

Scientists have detected large amounts of carbon gas in a newly forming solar system around Beta Pictoris, a nearby young star.
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Scientists have detected large amounts of carbon gas in a newly forming solar system around Beta Pictoris, a nearby young star.

The finding, detailed in the June 8 issue of the journal Nature, explains why the star's planet-forming debris disk is enshrouded in a thick cloud of gas. This is a mystery that has vexed scientists for years. According to theory, the gas shouldn't be there at all.

"The star's radiation should blow the gas away," said study team-member Aki Roberge of NASA Goddard Space Flight Center in Maryland.

But carbon has specific atomic quirks that render it immune to most of the energy pouring out of the star as light. Like a ghostly fog that no wind can disperse, the carbon-rich gas encircling Beta Pictoris is oblivious to the gale-like solar radiation blustering through it.

The new finding raises the possibility that in a few million years time, Beta Pictoris could be home to bizarre alien worlds that sound like something dreamed up by astronomers who've read too much science-fiction.

"If carbon-rich worlds are forming in Beta Pictoris, they might be covered with tar and smog, with mountains made of giant diamonds," said Marc Kuchner, an expert on extrasolar planets also from Goddard. Kuchner, who was not involved in the study, said that "life on such a planet is not implausible, but it certainly would be exotic."

Beta Pic
Beta Pictorus—or simply Beta Pic to astronomers—is located some 60 light-years away. It is nearly twice as large as our Sun and less than 20 million years old. Scientists had previously thought that the gas around the young star had a similar element composition to gas in our own solar system.

The reason the gas could resist Beta Pic's strong stellar gusts was that some hidden mass of hydrogen or other gas was acting to "brake" the outflow, according to the earlier thinking.

But the new finding, made by NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) and data from the Hubble Space Telescope, reveals that the braking material is actually carbon, which is very abundant in Beta Pic compared to other elements.

Carbon doesn't feel strong radiation pressure from Beta Pic's light the way that other heavier elements such as nickel and iron do. It absorbs most strongly in the far-ultraviolet range—a band that Beta Pic doesn't radiate much in.

Like science-fiction
Scientists think the carbon is formed during collisions of asteroids and comets in Beta Pic's debris disk with each other. For this explanation to work, however, the asteroids and comets would need to be very carbon-rich themselves, but so far the ones scientists have analyzed in our solar system don’t fit this criterion.

This leaves two possibilities, scientists say. The first is that the asteroids and comets around Beta Pic contain more carbon-rich materials such as graphite and methane than the ones in our solar system.

This could have interesting consequences for future planets that might form around Beta Pic. Rocky bodies such as comets and asteroids are thought to be the building blocks of terrestrial Earth-like planets. According to the standard "core accretion" model of planet formation, these rocky bodies collide and clump together, growing larger and larger until they reach planethood.

If the planetary building blocks in Beta Pic are fundamentally different from those that existed in our early solar system, then any planets forming there will also be different—perhaps radically so.

"It could eventually lead to the creation of very exotic planets, like those in the realm of science fiction almost," Roberge told

In addition to diamond mountains, such planets might also have methane atmospheres like Saturn's moon Titan.

Another explanation
The second possibility is that the asteroids and comets around Beta Pic only appear exotic because they're young. According to this scenario, the asteroids and comets in our own solar system were also carbon-rich long ago but their compositions changed over time due to some unknown process.

This would mean that Beta Pic's solar system is a close analogue of what our early solar system was like and that future planets forming there might one day resemble the ones here.

"We might be observing processes that occurred early in our solar system's development," said study team-member Alycia Weinberger of the Carnegie Institution of Washington.

This scenario has interesting implications for the evolution of life on Earth, the researchers say.

Scientists think that asteroids and comets bombarded early Earth for the first few million years after it formed. This constant pummeling was a good thing in the long run because it delivered virtually all of the water and organic material to Earth that makes life possible.

If comets and asteroids contained more carbon during our solar system’s infancy than they do now, then more organic material might have been delivered to early-Earth than previously thought.

Roberge said that scientists will have to await the deployment of some future ultraviolet space telescope to test which of these two explanations are true. The Atacama Large Millimeter Array, a ground-based radio telescope scheduled for completion in 2012 might also work, she said.