Oort cloud and Scattered Disc formation during a late dynamical instability in the Solar System

•We did numerical simulations forming the Oort cloud (OC) and Scattered Disc (SD).•We incorporated giant planet migration in the framework of the Nice model.•We performed a size-based comparison between the OC and SD populations.•The OC to SD population ratio from observations is 44; from simulations it is 12.•These agree within error bars. The SD is more massive than previous estimates.

One of the outstanding problems of the dynamical evolution of the outer Solar System concerns the observed population ratio between the Oort cloud (OC) and the Scattered Disc (SD): observations suggest that this ratio lies between 100 and 1000 but simulations that produce these two reservoirs simultaneously consistently yield a value of the order of 10. Here we stress that the populations in the OC and SD are inferred from the observed fluxes of new long period comets (LPCs) and Jupiter-family comets (JFCs), brighter than some reference total magnitude. However, the population ratio estimated in the simulations of formation of the SD and OC refers to objects bigger than a given size. There are multiple indications that LPCs are intrinsically brighter than JFCs, i.e. an LPC is smaller than a JFC with the same total absolute magnitude. When taking this into account we revise the SD/JFC population ratio from our simulations relative to Duncan and Levison (1997), and then deduce from the observations that the size-limited population ratio between the OC and the SD is 44-34+54. This is roughly a factor of four higher than the value 12 ± 1 that we obtain in simulations where the OC and the SD form simultaneously while the planets evolve according to the so-called ‘Nice model’. Thus, we still have a discrepancy between model and ‘observations’, but the agreement cannot be rejected by the null hypothesis.