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Planetesimal belts are discovered around beta Pictoris
--revealing an early extrasolar planetary system

Illustration : Kouji KANBA

Beta Pic is a young main-sequence star with an edge-on circumstellar disk supposed to embody an aspect of the early solar system. Its dust is considered not to be remains from the protoplanetary disk but must be replenished by planetesimal collisions and/or evaporation from comets, though the detailed mechanism is still controversial. A team of astronomers from Ibaraki University, the Japan Aerospace Exploration Agency, the University of Tokyo, and the National Astronomical Observatory of Japan has observed the dust disk of beta Pictoris using the Subaru telescope and an instrument called the Cooled Mid-Infrared Camera and Spectrometer (COMICS). By analyzing the infrared dust band emission, the team discovered that small amorphous silicate grains have their distribution peaks around 6, 16, and 30AU (1AU=1.5x108km=the distance between the earth and the sun). Since the small grains are blown-out very quickly from the system due to the stellar radiation pressure on the dust, the team concluded that the grains are replenished at ring-like locations around 6, 16, and 30AU, which are conceivable as ring-like distributions of planetesimals, or 'planetesimal belts', like the asteroid belt in our Solar system. The beautiful illustration shows an imaginary picture of the planetesimal belts and the aspect that the grains are replenished by collisions between the planetesimals.

Current results have revealed inner structure of a forming planetary system by probing the locations of grain replenishment. In particular the regions probed by the team here correspond to the radius where planets exist in our Solar system. Detailed structure of such inner region has been revealed for the first time thanks to the mid-infrared high-resolution observations. The results here have a large impact on the study of planetary system formation. Perhaps, our Solar system has shown a similar aspect and frequent collisions of planetesimals have occurred at its young age, too. The results here have been published as a paper in Nature in October 2004.

Reference. Okamoto, Y. K. et al. Nature, Vol. 431, pp.660-663 (2004)

October 21, 2004