ACCORDING to conventional wisdom(1-4), the ring around supernova 1987A is a product of winds from tile progenitor star, which should have produced a thin, dense, spherical shell(1). It was accordingly a surprise when images obtained by the Hubble Space Telescope(5-8) revealed that the gas is in fact disposed in a thin ring, with a radial velocity(9) much smaller than that predicted by theory. This could be explained by an asymmetry in the red giant wind(10-12), or by rotational flattening(13,14), but these explanations seem to us to be ad hoc, and have associated problems. Here we propose, instead, that the ring is the inner rim of a disk of gas that is left over from the time of formation of the progenitor star. The centre of the disk was evaporated by the ionizing radiation of the progenitor over its lifetime of about ten million years, leaving a ring-like structure. Our hypothesis naturally explains the ring’s physical properties, and leads to the prediction that we should see the rest of the disk shortly after the supernova ejecta hit the ring in AD 1999 +/- 3.