ESA Science & Technology - Herschel image of Fomalhaut and its debris disc

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 micron. 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 kilometre 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 10¹¹ and 10¹³, 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.