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Monday January 7 4:27 PM ET

Astronomers Find Brown-Dwarf Orbit

By PAUL RECER, AP Science Writer

WASHINGTON (AP) - Astronomers have captured a direct image of a massive, planet-like object called a brown dwarf in close orbit of a distant star very much like the sun.

Michael Liu, a University of Hawaii astronomer, said the brown dwarf orbits around its parent star at slightly less than the distance between the sun and the planet Uranus. The new discovery orbits closer to its parent star than any other brown dwarf yet discovered, said Liu.

Using new technology that sharpens the view of ground-based telescopes, astronomers found the brown dwarf orbiting about 1.3 billion miles from the star known as 15 Sge in the constellation Sagitta, located about 58 light years from Earth. Uranus orbits the sun at about 1.7 billion miles. A light year is the distance light will travel in a year, about 6 trillion miles.

``This discovery implies that brown dwarf companions to average, sun-like stars exist at a separation comparable to the distance between the sun and the outer planets in our solar system,'' Liu said. He announced his discovery Monday at the national meeting of the American Astronomical Society.

Liu said the parent star of the brown dwarf is ``basically the same as our sun. It's like a clone of our sun, a solar twin.''

Finding the brown dwarf so close to the star suggests that solar systems formed around sun-like stars could come in many different shapes and planetary distributions. Liu said it is possible that there are planets similar to Earth circling the star inside the orbit of the brown dwarf. For instance, it would be possible for a planet to orbit the star at 93 million miles, the distance from Earth to the sun.

A brown dwarf, sometimes called a ``failed star,'' is bigger than a planet but much smaller than a star. They are thought to be balls of gas that failed to collect enough mass to start shining. Astronomers believe that a stellar object must have at least 8 percent of the mass of the sun for there to be enough internal pressure to start the nuclear fires that cause stars to shine.

Liu said the discovered brown dwarf is 55 to 78 times the mass of Jupiter, the largest planet in the solar system. Jupiter is 318 times more massive than Earth.

Finding a brown dwarf in close orbit of a sun-like star is a rare event. The objects are difficult to see because of the glare of the central star. Astronomers compare the feat to detecting a firefly next to a searchlight. In the past, ground-based telescopes were unable to make such observations because light arriving from distant stars is distorted by the Earth's atmosphere.

Liu said the orbiting brown dwarf was found using a new telescope technique called adaptive optics. The system, installed on two telescopes in Hawaii, use a flexible mirror that is constantly being bent slightly to compensate for the distortion of the atmosphere.

``Only by using adaptive optics to produce very sharp images could we have found this companion,'' said Liu. ``It is too faint and too close to its parent star to be seen otherwise.''


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