A group of four astronomers has refined a theory of how planets form within disks swirling around stars. They say that over time, would-be globes form within narrow rings that migrate from the inside to the outside of dusty gas disks, depositing planets along the way.
The research pushes astronomers, who first succeeded in locating solar systems beyond ours just four years ago, to adjust their search for new planetary systems by looking not only for thin pancakes of dust circling stars, but also for rings of dust within them.
"You can think of planet forming in the disk as a wave that goes outward," said Scott Kenyon, an astronomer at the Harvard-Smithsonian Center for Astrophysics who headed up the research published in the latest issue of Astrophysical Journal Letters.
"And it takes several tens of millions of years to go from the inner edge near the star to the outer edge of the disk. If you could make a time lapse movie of planet formation in one star, youd see successive rings close the star and then propagating outward."
Kenyon and his colleagues built a computer model of planet formation based on observations made in 1998 the Hubble Space Telescope to study a ring of dust orbiting the star HR 4796A, which scientists suspect harbors a young planetary system.
Their model suggests that the dust ring around that star, thought to be about 10 million years old, is probably a common feature of all planetary systems. For instance, the Kuiper Belt of asteroids around our solar system may be the rocky remains of such a planet-forming disk from 5 billion years ago, Kenyon said Monday. There may be several Pluto-like planets hiding out in that belt waiting to be discovered, he added.
Many astronomers think that planets like Earth, Mars and Jupiter grow when micro-planets coalesce in a thick disk of dust and gas around a hot, young star. Because the dust initially reflects light from the central star, powerful telescopes can see the disk.
The so-called planetesimals collide constantly and eventually stick together like snowballs to form objects large enough to be thought of as planets in a process that can last up to 100 million years.
Kenyon's model showed that planet-spawning dust around stars clusters in rings more than it does in flat disks, with the ring moving from close to the star to the last stop on the line, marking the present site of planet formation.
The amount of light reflected from the ring can be used to measure the vast number of dust grains within and therefore the amount of material available to form planets, as well as the amount that might form. Rings that are more massive generate more collisions and therefore, eventually form more planets, Kenyon said.
"The ring is where all the action is happening," he said. "I suspect that you have a ring like this at the edge of every star that makes planets. It either looks like a ring or the Kuiper Belt."
Geoff Marcy, an astronomer at the University of California at Berkeley, heads up the team that has found 20 of the 28 so-called extra-solar planets currently catalogued.
"What's extraordinarily exciting here is that this proto-planetary disk actually is not a disk, as we all imagined," Marcy said Tuesday. "It's a ring like the one on your finger. It means that somehow the gas and dust orbiting this star is confined to a little narrow strip going around the star some 70 Earth-sun distances from the star."
Astronomers have yet to directly observe the typically dim planets orbiting suns beyond ours, as the presumed planets are lost in the glare of their central stars. Instead, astronomers in the past few years have used an inferential technique to find extra-solar planets by detecting the gravitational tug or wobble they exert on their central stars.
Recent innovations in studying disks around stars using powerful infrared telescopes, including the Hubble Space Telescope, have generated a new sub-field in astrophysics in the past few months, Marcy said. Now astronomers are studying disks and rings around stars thought to be generating planets to learn their temperature and density and use that information to learn more about planet formation, he said.
"Up until two to three years ago, we had no high-resolution infrared imaging abilities to see planets caught in the act of forming. Now that's possible," Marcy said. "It's an entirely new way to characterize planets around stars without working so hard to actually detect planets themselves."
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