Photo Suggests Planet Under Construction Around Star

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Published March 28, 2008

| Space.com

Astronomers have peered into the womb of a stellar disk to capture an image of material falling onto what could be a planet in an early stage of formation.

The new image shows a somewhat horseshoe-shaped void in the disk surrounding a young star called AB Aurigae about 470 light-years away.

Within the void, a barely visible bright spot could indicate a developing object that's currently between 5 and 37 times the mass of Jupiter.

"The deficit of material could be due to a planet forming and sucking material onto it, coalescing into a small point in the image and clearing material in the immediate surroundings," said researcher Ben Oppenheimer, an astrophysicist at the American Museum of Natural History in New York. "It seems to be indicative of the formation of a small body, either a planet or a brown dwarf."

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A brown dwarf is considered a star that's not massive enough to generate the thermonuclear fusion that powers real stars.

To date, no confirmed extrasolar planet has been imaged or seen directly, as no one yet has succeeded in effectively blocking the light from the parent star that overwhelms the faint glow of a nearby planet.

If this object is a planet, the image does not show the planet itself. Oppenheimer said the image shows what's thought to be dust accreting onto the object.

"The main problem is that the stars are hundreds of millions to billions of times brighter than the planets that orbit them," Oppenheimer told SPACE.com. "So the glare of the star wipes out any hope of really seeing the planets."

Planet-formation theorist Alan Boss of the Carnegie Institution of Washington called the finding "an intriguing situation." He added: "And given that it's a fairly massive disk with big spiral features, there's a good chance that something is forming in it."

Based on the apparent intensity of the object along with its distance from the primary star (about 100 times the distance from Earth to the sun), Boss speculates the object is more likely a brown dwarf.

Oppenheimer and his colleagues blocked out most of the stellar glare by attaching a coronagraph they had developed to a U.S. Air Force telescope on Maui, Hawaii. They also used polarization filters, which show light scattered off the disk.

The observations, set to be detailed in the June issue of the Astrophysical Journal, could fill a gap in astronomers' understanding of planet formation, which they are pretty confident occurs within the disks of material surrounding young stars.

While the exact ways in which gas giant planets and brown dwarfs form are not known, it's possible both objects develop in the same manner in the material that swirls around a newborn star.

Boss calls the finding a "great step forward toward trying to understand how planets form and being able to image planets in formation as well as mature planets."

The star AB Aurigae is quite young, estimated to be between 1 million and 3 million years old.

Our sun, by comparison, is 4.6 billion years old, but most of the planet formation in our solar system is thought to have taken place in the first hundred million years or so.

Past observations of stars slightly older than AB Aurigae indicate that at some point during planet formation, gas is removed from the dusty disk surrounding the stars. How the gas exits has remained a mystery.

The situation at AB Aurigae could represent an intermediate stage in which some mechanism is clearing out the gas from the disk's center and leaving behind mainly dust.

"The image produced speaks directly to the biggest, unresolved question of planet formation — how the thick disk of debris and gas evolves into a thin, dusty region with planets," said National Science Foundation Program Manager Julian Christou.

The research was funded in part by the NSF, U.S. Air Force, NASA's Terrestrial Planet Finder Program and individual donors.

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