Weekly Science Update

Friday, November 07, 2008
Studying a Young Solar System
When the first infrared cosmic survey satellite, IRAS, looked at the nearby star Epsilon Eridani in 1984, it found that the star emitted a large excess of cool infrared radiation. This star is only 10.5 light-years from earth, and had already been carefully examined at optical wavelengths. Those studies had showed that it is quite similar to our sun in mass, but is much younger -- only about 850 million years old versus the sun's age of 4.5 billion years. When the sun was as young as Epsilon Eridani we think it was in the process of forming its system of planets. The discovery of strong excess infrared emission, therefore, immediately suggested that the star had a disk of preplanetary dust around it, and this dust was the source of the excess infrared.

That conclusion has since been confirmed by other observations, and moreover other stars have been discovered with preplanetary, infrared-emitting disks around them. But Epsilon Eridani remains a pivotal example because it is near enough to us to allow close scrutiny. Last week a team of twelve astronomers including CfA astronomers Massimo Marengo, David Wilner, Tom Megeath and Giovanni Fazio announced the results of their combined infrared and submillimeter wavelength study of the dust disk in Epsilon Eridani. They used five different instruments to probe the nature of the emission. They find clear evidence that the disk consists of three separate rings: an "asteroid belt" similar to the one in our solar system about three astronomical units (AU) away from the star, a second "asteroid belt" about seven times farther out than the first and unlike anything in our solar system, and a third, previously-known icy ring of material at 35 to 100 AU away from the star with about 100 times as much material as is in our solar system's outer reservoir ring.

The new results also find gaps between these rings. These gaps were created, the scientists suggest, by the presence of otherwise unseen planets that cleared out the material in their orbits. The overall picture suggests a very early analog to our solar system in which three planets with masses between those of Jupiter and Saturn clear out rings in the young circumstellar disk. The disk is probably made mostly of silicate and ice dust grains that are short lived, and so must be constantly regenerated from collisions between larger, perhaps kilometer-sized, objects in the outer ring. This paper is an important step in our understanding of how the early solar system may have formed and evolved.

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