Astronomers Offer ‘Definitive’ Evidence in Star Studies : New Planetary System May Be Forming

Two astronomers have found a ring of dust and gas orbiting a young star in a way that suggests the early stages of planetary formation, strong evidence that there may be planetary systems other than ours in the universe.

In a major milestone in the search for life elsewhere, Steven Beckwith of Cornell University and Anneila Sargent of Caltech described their evidence as “the most definitive demonstration yet that other stars might have planetary systems like our own.”

The likelihood of other forms of life hinges partly on the question of how rare or common are planetary systems that could support life. The two astronomers said they believe the evidence suggests that such systems may be much more common than had been thought.

Beckwith and Sargent made their announcement during the annual meeting of the American Astronomical Assn. in Pasadena.

What they have discovered is a ring of dust and gas orbiting a distant star that is very much like the Earth’s sun in the days of its youth; they say the ring of gas is behaving just as it would if planets were being formed around the star.

“This is the first gas and dust that fits all the predictions” that would have to be met if planets were being formed around another star, Beckwith said.

“It’s got all the potential,” Sargent added.

Current thinking on the formation of planets holds that young stars will be surrounded by some of the material from which they were formed. As that material orbits the star, it is slowly drawn into a flat disk by the influence of the star’s gravitational field.

Over millions of years, dust particles will bang into each other as they orbit the star, and some of them will stick together. As these particles grow, they develop their own fields of gravity and begin to slowly gather other particles. Eventually the dust particles grow into rocks and the rocks turn into planets, according to the theory.

Beckwith and Sargent used a sophisticated array of three radio telescopes at Caltech’s Owens Valley observatory in determining that the gas and dust closest to the star is rotating around the body faster than materials farther out in the gaseous disk--just as Johann Kepler had predicted when he postulated Kepler’s Law in the 17th Century to define the motion of planets around the sun.

The two made that determination by analyzing the varying wavelengths of electromagnetic radiation emitted by carbon monoxide within the ring of gas and dust.

The Owens Valley facility is especially sensitive to emissions from molecular gas, such as carbon monoxide, thus permitting Beckwith and Sargent to determine relative orbiting speeds of gas close to the star and gas farther out. This can be done because wavelengths are stretched whenever the source is moving away from the observer but are condensed whenever the source is moving toward the observer.

To confirm the theory of planetary formation, Beckwith and Sargent began searching for stars that are about the age the sun was when the planets were formed about 4 billion years ago. They studied half a dozen stars and eventually focused on HL Tauri, a nascent star in the constellation Taurus that is about 500 light-years from Earth. A light-year is the distance light travels in a year, nearly 6 trillion miles.

It is particularly important that HL Tauri confirmed the predictions because it is one of only a few stars the two astronomers examined.

‘Reasonably Common’

“The formation of planets must be reasonably common or we wouldn’t have stumbled across it so quickly,” Beckwith said.

He and Sargent said they would consider planetary systems “common” throughout the universe even if only 5% of the stars have planets. That would greatly increase the chances of life occurring elsewhere because there are trillions upon trillions of stars.

“We’re pretty confident about it,” Sargent said.

Jill Tarter, an astronomer at the University of California, Berkeley, who has long been involved in the search for life outside this solar system, hailed the discovery as “terrific.” She cautioned, however, that the finding still falls short of proof that other planetary systems abound.

“We do know you need a disk to get planets. It’s necessary, but it’s not sufficient. I still don’t see the planets, nor could you,” she said.

The search for planetary systems has proven somewhat frustrating to astronomers because planets are far too small and distant to be seen, even with the most powerful telescopes.

The problem is further compounded by the fact that planets, being relatively close to their host star, are likely to be overwhelmed by the light from the star and thus be indistinguishable to telescopes.