Astronomers find
signs of planets
circling a starA UH scientist helps
By Paul Recer
discover what may be the
first multiple-planet system
found beyond our own
Associated PressAUSTIN, Texas -- Powerful new evidence for the presence of planets outside the solar system has been found in the first pictures ever taken of dust rings around distant stars.
Images captured by the Hubble Space Telescope clearly show dust rings around two stars and astronomers said such rings probably formed because the stars have planets.
The astronomers, including Bradford A. Smith of the University of Hawaii, announced the discovery yesterday 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.
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 ring around one of the stars, HD 141569, has a dark gap that may have been cut by the gravitational influence of a planet, said Alycia Weinberger of the University of California, 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 Smith, 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 gets big enough, its nuclear fires are ignited. The outflow from the star blows away the remaining 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 no planets are 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 4 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 5 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.
Images 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 4 miles away.
