Instead of planets taking millions of years to form as previously thought, researchers said new calculations suggest they sometimes can form within a few centuries.
|Artist’s conception of a gas giant planet orbiting a nearby star.
Credit: NASA and G. Bacon (STScI)
"The first one in our model pops up around 150 years," researcher Thomas Quinn, an astrophysicist at the University of Washington in Seattle, told United Press International. "Things can happen quickly."
When it comes to planetary formation, the standard theory says it takes a million years or more for the solid cores of gas giants such as Jupiter or Saturn to clump from the cosmic debris that whirls around young stars. After the cores appear, according to the theory, it takes another 1 million to 10 million years for envelopes of gas to enshroud them.
Recent surveys of some 1,000 stars reveal about 10 percent have gas giants orbiting them, generally ranging in size from about the mass of Jupiter to 10 times that large. "If they take millions of years to form, then they probably would be a very rare phenomenon," Quinn said.
The new model from Quinn and colleagues suggests the spinning disks of gas that orbit stars break apart after only a few spins. Fragments then quickly begin to coalesce due to gravity. "If this really happens out there, then it would probably dominate the way planets form," Quinn told UPI.
|Lalande 21185, a dim red dwarf, may have as many as three Jupiter-class
planets — including innermost planetary candidate "b" depicted with rings and two moons
Credit: John Whatmough
Although scientists have considered such a scenario for decades, the calculations involved have been forbidding. But in the Nov. 29 issue of the journal Science, Quinn and his team reported simulating one million clouds of gas, each one-thirtieth of an Earth mass, at one-hour intervals as they interacted gravitationally for up to 350 years.
"We used a fraction of the machine at the Pittsburgh Supercomputing Center (at Carnegie-Mellon University) for a solid several weeks. It’s roughly like having 100 of the fastest Pentium computers all running the same calculations for a couple of weeks," Quinn said.
Refining the calculations took nearly two years, he added. The results suggest gas giants can grow in fewer than 1,000 years, and accumulate masses similar to those spotted around other stars.
Planetary scientist Jack Lissauer of NASA Ames Research Center in Moffett Field, Calif., remains skeptical.
"It’s not that I think the calculations are bad. They’re the best calculations on this facet of the problem that have ever been done," he said. However, Lissauer said, for planets to form this quickly, the disks from which they emerge need to be very unstable, and before any forming planet reached that stage slight instabilities would cause spiral waves to flatten out any clumps.
Also, Quinn said, the computer model does not explain how rocklike planets such as Earth form, for which the standard model can provide an answer. Neither model yet can explain why so many planets seen outside our solar system orbit so close to their stars, he added.
|Protoplanet simulation from spinning disk of dust and solar system debris
Credit: Univ. Wash Seattle, Quinn
"Do I think this work is an important advance? Yes. Do I think it is definitive? No," said computational astrophysicist Richard Durisen of Indiana University in Bloomington. He and Quinn said future calculations need to take better account of the extremely complex role temperature plays.
"That’s actually quite hard," Quinn explained, noting it would require at least 2-to-10 times more computing power. "Fortunately, computers always get faster, so that makes it doable in a year’s time or whenever," he said.
In December 2001, NASA selected the Kepler Mission, a project based at NASA Ames, as one of the next NASA Discovery missions. The Kepler Mission, scheduled for launch in 2006, will use a spaceborne telescope to search for Earth-like planets around stars beyond our solar system. A key criterion for such suitable planets would be whether they reside in habitable zones, or regions sometimes protected by gas giants but with temperate climates and liquid water.
Future missions, such as ESA’s Herschel mission will search for many more and take detailed pictures of stars that might harbor dust rings. As these images become available, astronomers will be able to predict the sizes and orbits of giant planets within the alien solar system.
Citation: Lucio Mayer, Thomas Quinn, James Wadsley, and Joachim Stadel, "Formation of Giant Planets by Fragmentation of Protoplanetary Disks", Science, Nov 29 2002: 1756-1759