Crystal-Clear View of Planet Building

Building planets is easier than cooking a turkey: Take a lot of gas and dust, stir gently, allow clumps to form, then leave alone for 10 million years. The result is a young star surrounded by a small family of planets. At least, that's how our own Earth supposedly came to be, some 4.6 billion years ago. Now, astronomers have witnessed the onset of the process close-up for the first time.

University of Amsterdam astronomers Roy van Boekel and Michiel Min and their colleagues used the European Very Large Telescope array in northern Chile to focus on the cores of three young protoplanetary disks, surrounding stars a few hundred light-years away. By combining the power of two 8.2-meter telescopes, they were able to study the inner few hundred million kilometers of each disk--a region as small as Earth's orbit around the sun. The infrared radiation from these regions revealed copious amounts of micrometer-sized crystalline grains. In one case, the observed spectrum is consistent with 100% crystalline silicate, the team reports in the 25 November issue of Nature. These are exactly the types of grains scientists think are the building blocks for planets like Earth.

The discovery may also help solve a nagging problem regarding the makeup of planetary systems. Crystalline silicates, with individual atoms neatly arranged in lattices, are expected to form at high temperatures close to the star, but they also show up in comets--icy bodies that formed in the cool, outer parts of our solar system. How the silicates got there is a puzzle. Maybe turbulence in the disk took the grains outward, or maybe crystallization occurred in the outer disk under the influence of cosmic rays or lightning.

The new observations--the first in which researchers could separately probe the inner and outer parts of the disks--seem to support the turbulence scenario, says theorist Pawel Artymowicz of Stockholm Observatory in Sweden. The discovery of large reservoirs of crystalline material close to the stars suggests that similar grains in the cooler, outer parts of protoplanetary disks, where comets form, have somehow been transported outward, he says, maybe as a result of radiation pressure from the young star. "It's a beautiful result," says Artymowicz. "It's great to have observations pushing people into refining their theories."

Related sites
Introduction to the origin of planetary systems
MIDI instrument at the Very Large Telescope, used in the study