  Observations of atmospheric metals and dust discs around white dwarfs provide
important clues to the fate of terrestrial planetary systems around
intermediate mass stars. We present Spitzer IRAC observations of 15 metal
polluted white dwarfs to investigate the occurrence and physical properties of
circumstellar dust created by the disruption of planetary bodies. We find
subtle infrared excess emission consistent with warm dust around KUV 15519+1730
and HS 2132+0941, and weaker excess around the DZ white dwarf G245-58, which,
if real, makes it the coolest white dwarf known to exhibit a 3.6 micron excess
and the first DZ star with a bright disc. All together our data corroborate a
picture where 1) discs at metal-enriched white dwarfs are commonplace and most
escape detection in the infrared (possibly as narrow rings), 2) the discs are
long lived, having lifetimes on the order of 10^6 yr or longer, and 3) the
frequency of bright, infrared detectable discs decreases with age, on a
timescale of roughly 500 Myr, suggesting large planetesimal disruptions decline
on this same timescale.
