Planets and the Snow Line

As a new star develops within a molecular cloud, a circumstellar disk forms naturally from the rotating gas and dust. These disks are called "protoplanetary disks" because astronomers expect that much of their material will gradually coagulate to form planets. The farther away from the star the gas and dust is, the cooler it is, until at some critical distance molecules in the gas will condense out onto the dust grains. This process is believed to have played a critical role in the formation of planets in the solar system.

The distance at which a molecular species freezes out is termed its "snow line." Snow lines are thought to mark regions of enhanced particle growth -- and thus planet formation – for four reasons. There is more in solid material (versus gaseous material) beyond to the snow line from which to build grains; additional freezing-out can occur as gas diffuses across the snow line; solid grains can pile-up just inside of the snow line in pressure traps; and the grains onto which gas condenses become stickier with their icy mantles, favoring yet more coagulation.

Experiments and theory on these various processes have been focused on the water snow line, but the results should be generally applicable to the snow lines of other abundant volatiles too. Determining the snow line locations for various species is key to probing grain growth and planet formation efficiencies. When our solar system was young, for example, it is thought the water snow line developed at a distance of about three astronomical units (AU; one AU is the average distance of the Earth from the Sun.) The location of the water snow line was critical to the formation of Jupiter and Saturn because at farther, colder locations methane and carbon monoxide (CO) freeze-out enhanced the sold surface density, perhaps thereby contributing to the feeding zones of Uranus and Neptune. The latest solar system models also claim that most comets formed even farther out, at about thirty-five AU.

CfA astronomers Chunhua Qi and David Wilner and their colleagues have used new the giant new radio telescopes at the Atacama Large Millimeter/Submillimeter Array (ALMA) to image the locations of a simple nitrogen-bearing molecule that traces carbon monoxide (CO) depletion. (The presence of gaseous CO chemically inhibits the formation of this nitrogen species.) The scientists report finding that, in the young star TW Hya, the CO snow line lies at about thirty AU. This result is roughly consistent with some aspects of newer solar system models, and shows how new observations and theory are making dramatic strides in explaining how our solar system formed and evolved.

"Imaging of the CO Snow Line in a Solar Nebula Analog," Chunhua Qi, Karin Oberg, David Wilner, Paola d'Alessio, Edwin Bergin, Sean M. Andrews, Geoffrey A. Blake, Michiel Hogerheijde, and Ewine F. van Dishoeck, ScienceEXpress, July 29, 2013.