From Particles to Planets by Eng-Tips

by Kathleen M. Wong - ScienceMatters @ Berkeley

Eugene Chiang studies planet formation as a Berkeley professor of astronomy and earth and planetary science. Photo credit: courtesy Eugene Chiang

Dust, to most of us, is nothing more than a nuisance. The grayish film that collects atop bookshelves and beneath the couch is the bane of housekeepers from Arkhangelsk to Zimbabwe. But Eugene Chiang, a Berkeley professor of astronomy and earth and planetary science, says dust deserves more respect. It is, after all, the foundation upon which entire worlds are built.

A theoretical astrophysicist, Chiang studies how planetary systems form, “starting from disks of gas and micron-sized particles in orbit around young stars, and ending with congealed objects as massive as Jupiter,” he says.

How dust agglomerates into something the size of a planet remains a subject of hot debate. The trick, in terms of physics, is getting started. What mechanisms could drive specks of dust to clump in the first place? One possibility is by collisions. In this scenario, dust particles carom off one another until, by chance, a few begin to stick. But as anyone who has dropped one rock atop another knows, they easily rebound, chip, or shatter.

Chiang and collaborators numerically simulate how a dense disk of dusk surrounding a star will rotate faster than the layers of gas above and below it. The difference in speeds may trigger the Kelvin-Helmholtz instability—seen here in clouds—that might inhibit planet formation. Image credit: Mila Zinkova

Chiang sees a different mechanism at play. He envisions gravity gently pulling ensembles of grains together until they coalesce into giant masses. “Grains might settle towards the midplanes of disks into a thin and dense enough layer that they can self-gravitate into objects easily kilometers in size. You jump from microns to kilometers, and then you’re on your way to forming even larger objects,” Chiang says.

He and astronomy graduate student Aaron Lee are modeling the dust as a fluid that intermingles with the gas, and simulating the conditions under which self-gravity can overcome the turbulence in the gas. Turbulence is the enemy of planet formation because it keeps small particles stirred up, but Chiang has determined that disks sufficiently laden with dust can forestall such turbulence.

A solar system one step closer to forming planets no longer has a continuous disk of dust around its host star. Instead, it has an outer dust ring and a relatively transparent, dust-free center.

From this and other equally grainy images, Chiang and collaborators deduced both the orbit and the mass of the planet Fomalhaut b, as well as the presence of three Earth masses worth of boulder-sized objects in the dust ring. Image credit: Paul Kalas/UC Berkeley, NASA, ESA

Despite appearances, that hole is far from empty. “The star itself is radiating prodigiously at ultraviolet wavelengths, so there must be large amounts of gas crashing down upon its surface,” Chiang says. “Gas is funneled from the hole onto the star.” By rights, however, that gas supply should be unavailable. The centrifugal force caused by the rotation of the disk should hold both the gas and the dust in place.

In the 1990s, scientists discovered that magnetic fields could brake the rotation of the gas disk and allow gas to stream inward. Chiang is investigating whether this mechanism is at play in the centers of donut-hole systems.

Magnetic fields strongly influence charged particles such as electrons and ions but have no influence on uncharged materials such as wood. By considering a wide array of chemical reactions occurring within the disk, Chiang and physics graduate student Daniel Perez-Becker are calculating how much free charge exists in young solar systems, to assess the relevance of magnetic fields.

In his “Order of Magnitude” class, Chiang teaches graduate students how to estimate any quantity under the sun. The class tackles problems like the amount of plastic in the Pacific Ocean (roughly equivalent to the mass of surface plankton). “The point is to be bold and to not get bogged down with details,” he says. “Details can always wait until after you’ve made a brave first stab at the problem.” Photo credit: Algalita Marine Research Foundation

Of special concern is whether soot-like particles called polycyclic aromatic hydrocarbons, or PAHs, each containing several dozen carbon atoms, may keep the gas too neutral. Observed in disks, PAHs not only readily absorb free electrons, but also neutralize ions. Chiang and Perez-Becker are determining whether PAHs are abundant enough to leave the gas magnetically inert.

More fully formed solar systems hold equal allure for Chiang. In 2005, Berkeley astronomers Paul Kalas and James Graham spotted a planet circling a nearby star called Fomalhaut. Images of the planet consist of little more than a few pixels of light. Even so, Chiang was still able to deduce considerable information from them.

“We studied how the planet interacts with the belt of dust particles lying just outside the planet’s orbit. The more massive the planet, the more strongly it kicks the particles, and the more diffuse the belt becomes,” Chiang says. He and others used a similar principle to deduce the shape of Formalhaut b’s orbit: the more oval the ring of dust, the more eccentric, or egg-shaped, the planet’s path.

Chiang and earth and planetary science graduate student Edwin Kite used this data to infer that the planet may be half the mass of Jupiter. Extrasolar worlds previously imaged are all ten or more times the mass of Jupiter. Astronomers consider such massive objects brown dwarfs, not planets. Chiang’s findings establish Fomalhaut b as the first true planet to be sighted outside of our solar system—not so shabby for plain old dust.

This entry was posted on Wednesday, December 16th, 2009 at 5:42 PM and is filed under Community Manager. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.