EDP Sciences
Free access
Volume 492, Number 1, December II 2008
Page(s) 251 - 255
Section Planets and planetary systems
DOI http://dx.doi.org/10.1051/0004-6361:200810452
Published online 15 October 2008

A&A 492, 251-255 (2008)
DOI: 10.1051/0004-6361:200810452

Research Note

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

R. Brasser

Department of Physical and Environmental Science, University of Toronto at Scarborough, 1265 Military Trail, Toronto, ON, M1C 1A4, Canada
    e-mail: brasser_astro@yahoo.com

Received 23 June 2008 / Accepted 6 October 2008

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  $M_{\oplus}$. 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 ( $a \la 3000$ 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  $M_{\oplus}$ to 7.2  $M_{\oplus}$, with a value around 5  $M_{\oplus}$ the most probable. The mass in the outer Oort cloud ranges from 0.5  $M_{\oplus}$ to 1  $M_{\oplus}$, with a value close to 0.9  $M_{\oplus}$ being the most likely. Current estimates of the mass in the outer cloud based on observations range from 0.6  $M_{\oplus}$ to 1.4  $M_{\oplus}$, 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.

Key words: comets: general -- Oort cloud -- solar system: formation

© ESO 2008