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"There are many hazards that protoplanetary systems must survive to become planets," said John Bally, a professor of astronomy at the University of Colorado at Boulder. "My best guess is only 3 to 10 percent of stars can form solar systems like our own."

Planets form from disks of gas and dust encircling newborn stars. Tiny particles in these disks merge to form ever-larger clumps. The clumps that are close to their parent stars may form relatively small, rocky planets like Earth. Those that are farther out, where conditions are colder, can sweep up vast amounts of hydrogen and helium gas, forming giant planets like Jupiter.

But most stars are born in large, dense clouds of gas, where they're surrounded by many sibling stars -- and that presents a variety of hazards to potential planets.

Radiation pressure from high-mass stars, which are especially hot and bright, can blast away the disks of gas and dust from the other stars around them, quickly stripping those systems of the raw materials needed to form planets. And stars that are born with one or more gravitationally bound companions can lose their gas and dust as a result of interactions between stars; in essence, one star can "sweep away" the disk of material encircling another.

Rich stellar nurseries, which give birth to thousands of stars, may be especially troublesome areas for potential planetary systems. Such nurseries, like the Orion Nebula, a vast instellar cloud about 1,500 light-years away in the "sword" of Orion, typically contain several massive, hot stars, which pump out emormous amounts of radiation. This high-energy radiation acts like a blowtorch on the "protoplanetary disks" surrounding themselves and other stars, quickly blasting away their material.

Astronomers have discovered protoplanetary disks encircling dozens of young stars in the Orion Nebula, but in their hostile environment, few of the disks are likely to give birth to planets.

"The disks in the Orion Nebula are losing gas at such a rate that they'll be gone in a few thousand years," Bally said. "That implies that planets must form quickly, or they will be relatively rare....Something like 90 percent of the objects in Orion will lose their disks in something like 10,000 years or less."

This suggests that our solar system formed alone, far from the hot, massive stars that are disrupting planet formation in the Orion Nebula.

"We're coming in on a better understanding of the frequency with which solar systems like our own might be forming," said Mark Sykes, an astronomer at Steward Observatory in Arizona and chairman of the American Astronomical Society's Division for Planetary Science. "It seems like it's getting less and less."

At the same time, Sykes said that the star Zeta Leporis may harbor one or more planets similar to those in our solar system.

UCLA graduate student Christine Chen and her advisor, Michael Jura, reported that a relatively thick disk of dust surrounds the star, which is about 70 light-years away. The star is about twice as massive as our Sun, but only 1 to 10 percent as old -- about 50 million to 500 million years.

"This might be the first detection of a dust disk around a star comparable to our own solar system," Chen told reporters.

Astronomers already knew that dust surrounds Zeta Leporis, but Chen and Jura used the 400-inch (10-meter) Keck Telescope in Hawaii to study the star system in detail.

They found that material in the disk orbits Zeta Leporis at distances comparable to those of the inner planets and the asteroids in our own solar system.

Most important, individual particles in the disk should spiral into Zeta Leporis within about 20,000 years, implying that something steadily replenishes the supply of dust grains. Chen said the disk could be resupplied by the debris from collisions between larger bodies, such as comets or asteroids. The astronomers infer that the total mass of these parent bodies is several times greater than all of the asteroids in our own solar system.

Since planets probably form from the mergers of many smaller bodies, the findings suggest that one or more planets may orbit Zeta Leporis, although the researchers say they have yet to detect such a planet.

Damond Benningfield