We have used ISO, the Infrared Space Observatory, to perform a systematic s
earch for protoplanetary discs around nearby main-sequence stars. We find a
strong correlation with stellar age: more than half of the stars younger t
han 400 Myr do have such a disc, whereas less than 10% of the older stars e
xhibit disc emission. Such discs seem thus to decay on a timescale of a few
hundred Myr. Both the stability of the discs during this period and their
decay can be explained by collisions of planetesimals. Such collisions prod
uce the dust which is necessary to replenish the disc. The process stops as
soon as the planetesimals run out, either because they are all destroyed,
have escaped, or merged into planets, which implies the decay of the disc.
The timescale for the dissipation of protoplanetary discs is remarkably sim
ilar to the timescale of the heavy bombardment in our Solar System. The cra
tering of the Moon is a clear signature of the heavy bombardment which may
provide further fundamental clues to the formation of planetary systems.