ASTROPHYSICS: Silicon Seeding

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Science  02 Jun 2006:
Vol. 312, Issue 5778, pp. 1279d
DOI: 10.1126/science.312.5778.1279d

Why should planets form around some stars but not others? One clue has been that planets are more common around stars rich in iron. However, the role of iron in planetary growth remains unclear. Robinson et al. show that planet-hosting stars are enriched not only with iron but also with silicon and nickel. Silicon, in particular, may be a key player in the process. Initially, silicon is created from the fusion of oxygen nuclei inside the star, perhaps suggesting that planet-ringed stars should also be high in oxygen. Abundant silicon and oxygen could facilitate formation of a disk of solid debris around the star—indeed, silica and silicates are basic building blocks of most large planetary bodies in our own solar system.

The observed abundance of silicon also supports the core accretion model of the formation of large gas planets, such as Jupiter and Saturn. The hard cores of gas giants must grow rapidly, so that they can sweep in their gas atmospheres before the disk dissipates. To form a planet, the density of solid material in the disk must be high enough for the solids to clump together quickly. High silicon abundance would increase the likelihood of reaching this density threshold, perhaps helped by the presence of nickel and other heavy metals. — JB

Astrophys. J. 643, 484 (2006).

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