An alternative origin for debris rings of planetesimals

Abstract

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.Comment: Submitted to MNRA

Dawes Review 7: The Tidal Downsizing Hypothesis of Planet Formation

“…Bodies farther out are however unbound from the core when the gas is removed and are then sheared into debris rings with kinematic properties (e.g., mild eccentricities and inclinations) resembling the Kuiper and the Asteroid belts in the SS. The debris ring widens to R ∼ 20 AU at later times in the simulation (see Figure 3 in Nayakshin & Cha 2012).…”

“…Nayakshin & Cha (2012) argued that not only massive cores but also smaller, ∼1-1 000 km size bodies can be made inside the fragments. Analytical arguments supporting these ideas will be detailed in Section 7.3.…”

“…For central fragment density, an order of magnitude larger than the mean density, equation (10) of Nayakshin & Cha (2012) shows that R i ∼ 0.1R f , where R f is the fragment radius. Since the fragment is denser than the tidal density…”

“…Irregular satellites may be those solid bodies that orbited the solid core before the gas envelope of the parent gas fragment was destroyed. When the envelope is removed, the bodies that are weakly bound to the core obtain much more irregular orbits (Nayakshin & Cha 2012). …”

Towards a population synthesis model of self-gravitating disc fragmentation and tidal downsizing II: the effect of fragment–fragment interactions

“…Once disrupted, such bodies would potentially have produced planetesimal belts at the disruption radius, depending on the dynamical circumstances (Nayakshin & Cha 2012). This was never seen in the original Forgan & Rice (2013b) model runs, and seems to be possible only as a result of fragment-fragment interactions permitting fragments extra time to evolve before reaching their Roche limit.…”