Planetary Systems May Be Rare, Astronomers Report
(From the September/October 2001 issue of StarDate magazine)
Planetary systems like our own may be rare, suggesting that Earth and its inhabitants owe their existence to an unusual set of circumstances.
“There are many hazards that protoplanetary systems must survive to become planets,” John Bally, a professor of astronomy at the University of Colorado at Boulder, said during the American Astronomical Society (AAS) conference in early June. “My best guess is only three to ten 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 groups, so they are surrounded by many sibling stars — and that presents a variety of hazards to potential planetary systems.
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. The high-energy radiation from these stars 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 AAS Division for Planetary Science. “It seems like it’s getting less and less.” — Damond Benningfield
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