The preceding chapter reviewed the observational properties of the discs out of which planets are expected to form. The formation timescales are up to a few million years for gas giants, and tens of millions of years for terrestrial planets (based on the history of solar-system bodies). These durations occupy less than 1 % of the main-sequence lifetime of a star like the Sun (4.5 Gyr old now, age around 10 Gyr when it evolves into a red giant). Thus it might be expected that circumstellar discs are a phenemonon only of very young stars – the orbiting dust and gas being absorbed into planets or dispersed. In fact, very low-mass discs of dust particles are now known to exist around main-sequence stars, and as late as many Gyr (possibly also surviving into the giant phases).