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Fomalhaut is a young, nearby star that is suspected to harbor an infantplanetary system, interspersed with one or more belts of dusty debris. Wepresent far-infrared images obtained with the Herschel Space Observatory withan angular resolution between 5.7 and 36.7 arcsec at wavelengths between 70 and500 micrometer. The images show the main debris belt in great detail. Even athigh spatial resolution, the belt appears smooth. The region in between thebelt and the central star is not devoid of material; thermal emission isobserved here as well. Also at the location of the star, excess emission isdetected.  We use a dynamical model together with radiative-transfer tools to derive theparameters of the debris disk. We include detailed models of the interaction ofthe dust grains with radiation, for both the radiation pressure and thetemperature determination. Comparing these models to the spatially resolvedtemperature information contained in the images allows us to place strongconstraints on the presence of grains that will be blown out of the system byradiation pressure. We use this to derive the dynamical parameters of thesystem.  The appearance of the belt points towards a remarkably active system in whichdust grains are produced at a very high rate by a collisional cascade in anarrow region filled with dynamically excited planetesimals. Dust particleswith sizes below the blow-out size are abundantly present. The equivalent of2000 one-km-sized comets are destroyed every day, out of a cometary reservoiramounting to 110 Earth masses. From comparison of their scattering and thermalproperties, we find evidence that the dust grains are fluffy aggregates, whichindicates a cometary origin. The excess emission at the location of the starmay be produced by hot dust with a range of temperatures, but may also be dueto gaseous free-free emission from a stellar wind.

Herschel images of Fomalhaut. An extrasolar Kuiper Belt at the height of its dynamical activity