  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.
