Orbital Evolution of Planetesimals by the Galactic Tide

The Oort cloud is a spherical comet reservoir surrounding the solar system. There is general agreement that the Oort cloud comets are the residual planetesimals of planet formation. The standard scenario of the Oort cloud formation consists of two dynamical stages: (1) giant planets raise the aphelia of planetesimals to the outer region of the solar system and (2) the galactic tide, passing stars, and giant molecular clouds pull up their perihelia out of the planetary region.

Here we show the orbital evolution of planetesimals by the galactic tide. Planetesimals with large aphelion distances change their perihelion distances toward the outside of the planetary region by the galactic tide and become members of the Oort cloud. The effect of the galactic tide on the planetesimals with semimajor axes of ∼ 10⁴AU is about 10^-3 of the solar gravity. The timescale of the orbital evolution is ∼ 10⁸ years. We consider only the vertical component of the galactic tide. Under the axisymmetric potential, some planetesimals may show the librations around ω (argument of perihelion)=π /2 and 3π /2 (the Kozai mechanism). The alternate increases of eccentricity and inclination of the Kozai mechanism are effective to form the Oort cloud. The secular perturbation theory demonstrates the Kozai mechanism and we can understand the motion of the planetesimals analytically.

We apply the Kozai mechanism to the galactic tide and discuss the property of the Oort cloud formed by the Kozai mechanizm.

This work was supported by the 21st Century COE Program Origin and Evolution of Planetary Systems of the Ministry of Education, Culture, Sports, Science, and Technology, Japan, and JSPS Research Fellowship for Young Scientists.