A team led by the University of Colorado at Boulder has observed the first phases of planetary growth by confirming that tiny particles orbiting in dusty disks surrounding several young sun-like stars in the Orion Nebula are clumping together and growing.
But the same systems in which these observations were made are being rapidly destroyed by the violent environment, the researchers say. Given new observations and theories of how such systems evolve, they wonder if the planets will win out or their first steps of growth will be negated as they are torn apart by nearby star radiation.
Made with the NASA-European Space Agency聮s Hubble Space Telescope, the CU observations indicated dust bits about the size of smoke particles had accreted, or clumped together, into pieces about 1,000 times the mass of one particle, said former CU-Boulder research associate Henry Throop. Throop, who recently accepted a position at the San Antonio-based Southwest Research Institute聮s Boulder Office, said the dust particles stick together like a snowball picking up snow as it is rolled along the ground.
Hubble was used to look at the dust rings surrounding several sun-like stars in Orion in the visible and near-infrared wavelengths of light, said Throop. The background light from the Orion Nebula bounces off different-sized dust particles, scattering the light in a way that allows the researchers to estimate particle sizes from their observations.
"The dust we are seeing in the visible light observations appears gray, which we haven聮t seen in astronomy before," said Throop. "Small particles in space usually cause the light behind them to appear slightly red. That isn聮t happening here at all, and these dust grains are gigantic compared to what we expected to see."
A paper on the subject by Throop, Bally, CU-Boulder Professor Larry Esposito and Mark McCaughrean of Germany聮s Potsdam Astrophysical Institute will be published electronically on April 26 by the weekly journal, Science.
When a star forms, a giant gas cloud contracts into a titanic ball, causing the star to eventually "turn on." Usually, a large, doughnut-shaped dust disk is left circling the birthing star, said Throop. The researchers聮 models show that some particles in Orion聮s dust disks could grow to meter-sizes fairly quickly 聳 in about 100,000 years.
In active star nurseries like Orion, large and powerful type O or type B stars in the region make planetary formation around smaller, sun-like stars difficult, said John Bally of CU-Boulder聮s Center for Astrophysics and Space Astronomy.
"UV light comes streaming off these large stars like a blowtorch, evaporating the gases and removing the dust from the circumstellar dust rings of the smaller stars. Thus, massive stars are hazardous to planets forming around nearby, lower mass stars."
Orion is well known for its star formation, having produced roughly 20,000 low-mass stars like the sun in the last 10 million years. But the star birth process in Orion also has spawned at least a dozen high mass type O and type B stars that emit large amounts of UV light that may prevent most circumstellar disks from birthing planets.
"It is a mixed picture," said Bally. "We are seeing the destruction of most dust disks circling the smaller stars by UV light. At the same time, a few disks in the quieter, outer parts of the nebula show evidence of significant grain growth."
The research team estimated from models that it takes about 100,000 years for larger type O and B stars to remove the gas and dust from the disks ringing the smaller stars. "But the models show that at the inside edge of some disks, grains grow very rapidly to the size of gravel," said Throop. "If the particles reach about a centimeter to a meter in diameter, they will survive the destruction of the disk and may proceed at a leisurely pace to eventually form small rocky planets or asteroids."
Although regions like Orion often are referred to as "stellar nurseries," they actually are violent, destructive places because of the UV light produced by the massive stars. "It is a hard place to raise a family of planets," said Throop.
Throop doubts Jupiter-like planets can form in Orion because such gas giants require the mass of several rocky planets and enough gas to form the planet聮s exterior. He also doubts that Orion could contain icy planets or moons like those in the Kuiper Belt far outside our solar system, because the UV radiation would destroy the ice and remove the remaining small dust grains.
The team combined Bally聮s expertise in star and solar system formation with Esposito聮s and Throop聮s research on the evolution of planetary rings.
"The dynamics of planetary rings in our solar system are quite similar to the circumstellar dust rings we see in Orion," Esposito said. "I think it is likely there are other solar systems in the Milky Way, and astronomers have discovered giant planets around 5 percent of the stars examined near the sun."
If one or more of Orion聮s circumstellar disks around sun-like stars produces planets, they probably will be far different from planets in Earth聮s solar system, said Throop. "I think we can safely conclude that planetary systems, wherever they form, will be unique."