  The persistence of planetary systems after their host stars evolve into their
post-main sequence phase is poorly constrained by observations. Many young
white dwarf systems exhibit infrared excess emission and/or spectral absorption
lines associated with a reservoir of dust (or planetesimals) and its accretion.
However, most white dwarfs are too cool to sufficiently heat any circumstellar
dust to detectable levels of emission. The Helix Nebula (NGC 7293) is a young,
nearby planetary nebula; observations at mid- and far-infrared wavelengths
revealed excess emission associated with its central white dwarf (WD 2226-210).
The origin of this excess is ambiguous. It could be a remnant planetesimal
belt, a cloud of comets, or the remnants of material shed during the
post-asymptotic giant branch phase. Here we combine infrared (SOFIA, Spitzer,
Herschel ) and millimetre (ALMA) observations of the system to determine the
origin of this excess using multi-wavelength imaging and radiative transfer
modelling. We find the data are incompatible with a compact remnant
planetesimal belt or post-asymptotic giant branch disc, and conclude the dust
most likely originates from deposition by a cometary cloud. The measured dust
mass, and lifetime of the constituent grains, implies disruption of several
thousand Hale-Bopp equivalent comets per year to fuel the observed excess
emission around the Helix Nebula's white dwarf.
