A two-stage formation process for the Oort comet cloud and its implications

Abstract

Aims. The aim is to check whether the Nice model – which refers to an initially compact configuration of the giant planets that lasted for about 700 Myr and eventually became unstable when Jupiter and Saturn crossed their 2:1 mean motion resonance – is consistent with what we know of the Oort cloud, and to provide an explanation for the mass and structure of the Oort cloud. Methods. A two-stage formation scenario of the Oort cloud is proposed. The starting condition is the Nice model of planet migration, which contains a disc outside Neptune with a mass of 35 M⊕. From the formation efficiency I calculate the amount of mass deposited in the outer Oort cloud (comets with semi-major axis a > 20 000 AU; the inner Oort cloud has comets with semi-major axis a < 20 000 AU) and the innermost Oort cloud (a <∼ 3000 AU). The latter formed when the Sun was part of its birth cluster i.e. before the outer cloud. Results. The mass of the innermost Oort cloud directly inferred from the Nice model is 2.4 M⊕ to 7.2 M⊕, with a value around 5 M⊕ the most probable. The mass in the outer Oort cloud ranges from 0.5 M⊕ to 1 M⊕, with a value close to 0.9 M⊕ being the most likely. Current estimates of the mass in the outer cloud based on observations range from 0.6 M⊕ to 1.4 M⊕, in excellent agreement with predictions. Comets in the outer cloud come both directly from the disc outside Neptune as well as diffusion from the inner(most) cloud due to perturbations by passing stars.