Crystalline silicates in protoplanetary disks
Planets are formed by the accumulation of dust particles and gas molecules within the accretionary disks around stars. Based on observations of our own solar system it is evident that those particles include crystalline ferromagnesian silicates (olivine and pyroxene), quartz, metallic iron, troilite (FeO) and water. On the other hand, interstellar regions of our galaxy are dominated by amorphous (glassy) silicates, and not by crystalline silicates (Kemper et al., 2004).
Astronomers from a number of European countries have recently collaborated in a study of the accretionary disks around other stars in the galaxy, using the European Southern Observatory’s Very Large Telescope facility at Paranal, Chile. Their observations show that the dust within 2 AU (1 AU = 1 Earth radius) of three different stars is dominated by crystalline material with spectra that match those of olivine and pyroxene (see figure to the left) (van Boekel et al., 2004). In contrast, dust in the outer part of the disk (2 to 20 AU) includes some crystalline material, but is largely made up of amorphous silicates . Olivine to pyroxene ratios are close to 2:1 in the inner region (<2 AU) and around 1:1 in the outer region (2 to 20 AU).
The authors speculate that the crystallinity of these materials can be explained by thermal annealing (solid-state transformation from glass to crystalline structure) and by condensation of crystals from a gas phase. Since temperatures in the outer-disk regions are too low for thermal annealing, they speculate that the existence of some crystalline material in the outer disk must be a product of radial mixing.
The likely process of planet growth from particles and gas molecules in stellar accretionary disks is illustrated in the following diagram from Beckwith, Henning and Nakagawa (2000). In the early stages the dust sized particle aggregate by sticking and coagulation, and gravity does not play a significant role. Over a period in the order of 10,000 y particles can grow in size from microns to metres, and then over a similar length of time to planetismals with diameters of kilometres to tens of kilometres. At this stage graviational attraction, of particles with a wide range of sizes, becomes the dominant process. When the planetary diameter is in the order of thousands to tens of thousands of km its gravitational attraction is sufficient to sweep up gases in the vicinity of its orbit. The entire process probably takes in the order of millions of years.
Beckwith, S, Henning, T and Nakagawa, Y, 2000, Dust properties and assembly of large particles in protoplanetary disks, in Protostars and Planets IV, ed. by V. Mannings, A. P. Boss, and S.S. Russell, 533, Univ. Arizona Press, Tuscon.
Kemper, F, Vriend W and Tielens, A, 2004, The absence of crystalline silicates in the diffuse interstellar medium, Astrophysical Journal, V. 609, p. 826-837.
van Boekel, R and many others, 2004, The building blocks of planets within the 'terrestrial' region of protoplanetary disks, Nature, V. 432, p. 479-482 (Novmber 2004).
Steven Earle, 2004. Malaspina University-College, Geology Department, Return to Earth Science News