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An alternative origin for debris rings of planetesimals

Core Accretion, the most widely accepted scenario for planet formation, postulates existence of km-sized solid bodies, called planetesimals, arranged in a razor-thin disc in the earliest phases of planet formation. In the Tidal Downsizing hypothesis, an alternative scenario for formation of planets, grain growth, sedimentation and formation of planetary cores occur inside dense and massive gas clumps formed in the outer cold disc by gravitational instability. As a clump migrates inward, tidal forces of the star remove all or most of the gas from the clump, downsizing it to a planetary mass body. Here we argue that such a clump may form not only the planetary core but also numerous smaller bodies. As an example, we consider the simplest case of bodies on circular orbits around the planetary core in the centre of the gas clump. Bodies smaller than 1 km suffer a strong enough aerodynamic drag, spiral in and accrete onto the solid core rapidly; bodies in the planetesimal size range lose their centrifugal support very slowly. We find that planetesimals orbiting the protoplanetary core closely remain gravitationally bound to it; these may be relevant to formation of satellites of giant planets. Planetesimals on more distant orbits within the host clump are unbound from the protoplanet and are set on mildly eccentric heliocentric orbits, generically forming wide rings. These may correspond to debris discs around main sequence stars and the Kuiper belt in the Solar System. For the latter in particular, our hypothesis naturally explains the observed sharp outer edge and the "mass deficit" of the Kuiper belt.