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Monday, January 11, 1999 Dust rings around distant stars suggest presence of planetsAUSTIN, Texas (AP) -- The first telescope images of dust rings around distant stars reveal powerful new evidence of planets formed beyond the solar system, astronomers said Friday. Pictures from the Hubble Space Telescope of two distant stars show dust rings that may have been gravitationally sculpted by planets, astronomers said at the national meeting of the American Astronomical Society. The stars, identified as HR 4796A and HD 141569, are both about 300 light years from Earth. Each is twice the size of the sun, but much brighter and much younger at about 10 million years old. And, surrounding each star like immense hula hoops are disks of dust that appear to have been shaped by planets in the same way that the moons of Saturn sculpt the rings about that planet. The rings around one of the stars, HD 141569, have a dark gap that may have been cut by the gravitational influence of a planet, said Alycia Weinberger of the University of California at Los Angeles, a member of the astronomy team that made the discovery. "The most obvious way to form a gap in a disk is with a planet," Weinberger said at a news conference. "The disk is under the gravitational influence of some other body. This means that we have the circumstantial evidence of a planet about this star." The existence of such rings has been suggested in earlier studies, but the Hubble images are the first actual pictures, said Bradford A. Smith of the University of Hawaii, another team member. "Up until now, everything had been inferred," said Smith. "This is the first time we have actually seen a circumstellar disk. It came as a really big surprise. "When we saw it, we said, 'Wow, that looks like Saturn,"' he said. The formation of such rings is part of the process that builds a structure like our solar system. The process starts when dust and gas come together to begin forming a star. Some of the material also clumps together to form planets. When the star becomes big enough, its nuclear fires are ignited. The outflow from the star blows away the remaning dust and gas, leaving behind the planets and some orbiting boulders called plantisimals. The boulders collide and grind each other up, creating the dust. If there are no planets orbiting the star, the dust will disperse. But the presence of planets gives the dust a gravitational kick that shapes it into a disk. In the solar system, which is about four billion years old, only a faint dust ring remains. Around HD 141569, the disk is 75 billion miles across, ringing an area much larger than the entire solar system. The gap in the disk is prominent, occupying about five percent of the entire ring. The ring is about 21 billion miles from the star. The ring around HR 4796A is about 1.5 billion miles from its star. The ring is about 13 billion miles in diameter and about 1.6 billion miles thick. Imges of the rings were captured by a special camera on Hubble that allows photos of the area around a star while blocking the bright, obscuring light from its center. Smith said the rings are extremely faint: Detecting them is like trying to look at a dime from four miles away.
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