An alternative origin for debris rings of planetesimals

Core accretion (CA), the most widely accepted scenario for planet formation, postulates existence of similar to km-sized solid bodies, called planetesimals, arranged in a razor-thin disc in the earliest phases of planet formation. These objects coagulate by collisions, eventually building planetary cores. In the tidal downsizing (TD) hypothesis, an alternative scenario for the formation of planets, grain growth, sedimentation and planetary cores occurs inside dense and massive gas clumps formed in the outer cold disc by gravitational instability. As a clump migrates inwards, tidal forces of the star remove all or most of the gas from the clump, downsizing it to a planetary mass body. Here we consider a rotating and/or strongly convective gas clump. 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. We find that bodies smaller than similar to 1 km suffer a strong enough aerodynamic drag and thus spiral in and accrete on to the solid core rapidly. In contrast, bodies in the planetesimal and of a larger size range lose their centrifugal support very slowly. We consider analytically and numerically the fate of these bodies after the host gas clump is disrupted. Planetesimals orbiting the protoplanetary core closely remain gravitationally bound to it; these may be relevant to the 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, the TD hypothesis naturally explains the observed sharp outer edge and the mass deficit of the Kuiper belt.