The discovery of the first member of the Kuiper belt-a formerly hypothetical ancient reservoir of objects located beyond Neptune's orbit- started a revolution in our understanding of the outer Solar System: there is no longer a sharp edge at Pluto's orbit. About 60 Kuiper-belt objects, intermediate in size between comets and planets, are now known to exist on stable circular orbits around the Sun, and no doubt many more objects await discovery. But owing to the recent discovery and intrinsic faintness of these objects, little has been done to explore their physical and chemical properties. Here we report the results of a two-year survey of the broad- band optical colours of about one-quarter of the known Kuiper-belt objects. We find that their colours indicate the presence of two distinct populations: one consists of objects whose surface colours are only slightly redder than the colour of the Sun, while the other consists of the reddest objects known in the Solar System.
ASJC Scopus subject areas