The Moon and Extra-Solar Planets

We have used ISO, the Infrared Space Observatory, to perform a systematic search for protoplanetary discs around nearby main-sequence stars. We find a strong correlation with stellar age: more than half of the stars younger than 400Myr do have such a disc, whereas less than 10% of the older stars exhibit 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 produce 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 similar to the timescale of the heavy bombardment in our Solar System. The cratering of the Moon is a clear signature of the heavy bombardment which may provide further fundamental clues to the formation of planetary systems.