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Telescope Technology May Yield Images of Extra-Solar Planets

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By William Harwood
Special to The Washington Post
Tuesday, January 8, 2002; Page A03

Ground-based telescopes equipped with computer-controlled, flexible mirrors to counteract the blurring turbulence of Earth's atmosphere are on the verge of being able to photograph large planets orbiting nearby sun-like stars, astronomers reported yesterday.

While about 80 extra-solar planets have been discovered using indirect techniques -- measuring the effects of an unseen planet's gravity on the parent star, for example -- no one has directly made an image of a planet orbiting another sun.

But two discoveries reported yesterday at the winter meeting of the American Astronomical Society here indicate the rapidly maturing technology known as adaptive optics almost certainly will allow ground-based astronomers to do just that in a few years, if not sooner.

"It's technically now possible to directly image a young Jupiter around a nearby young star," said Ray Jayawardhana, an astronomer at the University of California at Berkeley. "We have not directly imaged a young planet yet. . . . But it could very well happen in the next few years, so keep your eyes and ears open."

With adaptive optics, a thin secondary mirror is precisely flexed many times per second under computer control so it can largely reverse the effects of atmospheric turbulence on light entering the telescope. The result is a much sharper image.

Jayawardhana and his colleagues used an adaptive optics system developed by the University of Hawaii and the eight-meter Gemini North telescope atop Mauna Kea to photograph what appeared to be a young double, or binary, star system in a small cluster of stars 900 light-years away.

The resulting image shows that one of the blurred "stars" in earlier ground-based photographs is actually two stars. A longer exposure shows yet another object near the freshly resolved pair of suns.

Remarkably, an adaptive optics close-up of the elongated object shows it to be an edge-on "proto-planetary disk" -- a flattened cloud of dust obscuring the parent star at the center of the disk. Reflected light from the star illuminates the object.

The stars making up this quadruple star system, and others in the cluster known as MBM 12, are thought to be about 2 million years old. The flattened disk indicates the process of planet-building is underway.

Using adaptive optics to obtain images of a broad range of young stars, astronomers hope to capture such disks in various stages of evolution, Jayawardhana said, "from the very beginning until it becomes a fully grown planetary system, to try to understand how long it takes, what processes are at work and whether it could happen anywhere and everywhere."

All of this leads to direct images of Jupiter-class planets from the ground -- no small feat when one considers that Earth's sun, when viewed across interstellar space in infrared light, would outshine Jupiter by about a billion to one. The trick is finding the planet in the glare of its sun.

"But a young Jupiter, because it's just formed and it's still hot and it's still contracting and actually emitting its own thermal radiation, would only be a hundred thousand times fainter than the young sun," Jayawardhana said.

"That may not seem like much of a gain to you, but it's 10,000 times brighter than an old Jupiter," he said. "And that is enough, already, for current adaptive optics systems on 10-meter-class telescopes to directly image a young planet."

Another team of researchers led by Michael Liu of the University of Hawaii provided a remarkable glimpse of the current state of the art -- and another glimpse of things to come -- in a related discovery.

Liu and his colleagues used adaptive optics with Gemini North and the huge, 10-meter Keck II telescope atop Mauna Kea to photograph a brown dwarf orbiting a sun-like star 58 light-years from Earth near an astronomical formation called the Summer Triangle.

Brown dwarfs are objects between 15 and 80 times the mass of Jupiter, not massive enough to trigger nuclear fusion in their cores. The brown dwarf discovered by Liu and company is thought to be about 65 times as massive as Jupiter.

It orbits its parent star at a distance of just 1.3 billion miles or so. In Earth's solar system, that would put it roughly midway between Saturn and Uranus. That's the smallest separation between a star and a "sub-stellar" object yet photographed, a feat made possible by adaptive optics.

Alan Boss, a leading planetary theorist at the Carnegie Institution, called the two discoveries "a very tantalizing appetizer for what's to come."

"They've now demonstrated they have the ability to" photograph Jupiter-class planets, he said. "There are several candidates out there -- it's just a question of time."

© 2002 The Washington Post Company

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