A New Type of Stardust--- Interplanetary dust particles contain rare grains that formed in stars older than the Sun.

Many meteorites contain small quantities of microscopic particles with unusual mixes of isotopes. The abundances of the isotopes, such as the relative amounts of oxygen-16, 17, and 18, indicate an origin outside the solar system. The dust particles are bits of stars, some of which no longer exist. Since their discovery in the late 1980s, only grains made of carbon (diamond and graphite), carbides, and oxides have been found. None were silicates--compounds that contain silicon, oxygen, and assorted other ions such as magnesium and calcium. This seemed peculiar to scientists because meteorites and the rocky planets are made mostly of silicate minerals. Part of the problem might have been a sampling bias introduced by the way grains of stardust were extracted, which involves dissolution of meteorites by strong acids. Silicates are more easily dissolved than carbides, oxides, or carbon compounds. However, Scott Messenger and his colleagues at Washington University in St. Louis and at the Johnson Space Center in Houston have found silicate grains in interplanetary dust particles, which are probably remnants of comets. This shows that presolar silicates exist, but it leaves open the question of why none have been found in meteorites