This image shows the thermal emission from the young star Fomalhaut and the debris disc surrounding it, as recorded with ESA's Herschel Space Observatory at a wavelength of 70 microns. Together with other data, this image suggests that the dust in Fomalhaut's debris disc consists of 'fluffy' aggregates: large conglomerates of small dust grains with lots of empty space in the structure.
Fluffy dust aggregates are believed to arise from collisions between comets. The radiation pressure exerted by the star, however, is expected to blow out small dust particles very efficiently, expelling grains from the disc immediately after they are produced by comet collisions. In order to explain the observed emission from Fomalhaut's debris disc, astronomers invoke a steady production of dust particles via comet collisions, with an average rate of 2000 daily collisions between comets with a size of one kilometer across or, alternatively, of 2 daily collisions between 10-kilometre-diameter comets.
The total mass of Fomalhaut's debris disc amounts to 110 times the mass of the Earth. This translates into a very large number of comets, ranging between 10^11 and 10^13, depending on their sizes. This is comparable to the mass of the primordial Kuiper Belt, which astronomers estimate to be of the order of 30 Earth masses.
Photo credit: ESA/Herschel/PACS/Bram Acke, KU Leuven, Belgium
Note: For more information, see Herschel Images Extrasolar Analogue of the Kuiper Belt; for a graphic that compares the size of the Fomalhaut planetary system to that of our own solar system, see Comparison of Fomalhaut's Debris Disc with the Solar System. Also, Herschel Spots Comet Massacre Around Nearby Star.
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