Low-temperature crystallization of silicate dust in circumstellar disks

Silicate dust in the interstellar medium is observed to be amorphous1, yet silicate dust in comets2,3 and interplanetary dust particles4 is sometimes partially crystalline. The dust in disks that are thought to be forming planets around some young stars5,6 also appears to be partially crystalline. These observations suggest that as the dust goes from the precursor clouds to a planetary system, it must undergo some processing, but the nature and extent of this processing remain unknown. Here we report observations of highly crystalline silicate dust in the disks surrounding binary red-giant stars. The dust was created in amorphous form in the outer atmospheres of the red giants, and therefore must be processed in the disks to become crystalline. The temperatures in these disks are too low for the grains to anneal; therefore, some low-temperature process must be responsible. As the physical properties of the disks around young stars and red giants are similar, our results suggest that low-temperature crystallization of silicate grains also can occur in protoplanetary systems.