Formation of Hot Planets by a combination of planet scattering, tidal circularization, and Kozai mechanism

TL;DR

This study explores how a combination of planet scattering, Kozai mechanism, and tidal circularization can efficiently form close-in giant planets, with about 30% success in simulations, highlighting the Kozai mechanism's key role.

Contribution

It demonstrates that the Kozai mechanism, combined with scattering and tidal effects, significantly increases the formation rate of close-in planets compared to previous models.

Findings

Approximately 30% of simulations produce close-in planets.

Kozai mechanism by outer planets induces high eccentricities leading to tidal circularization.

Close-in planets can have moderate eccentricities and a wide range of inclinations, including retrograde.

Abstract

We have investigated the formation of close-in extrasolar giant planets through a coupling effect of mutual scattering, Kozai mechanism, and tidal circularization, by orbital integrations. We have carried out orbital integrations of three planets with Jupiter-mass, directly including the effect of tidal circularization. We have found that in about 30% runs close-in planets are formed, which is much higher than suggested by previous studies. We have found that Kozai mechanism by outer planets is responsible for the formation of close-in planets. During the three-planet orbital crossing, the Kozai excitation is repeated and the eccentricity is often increased secularly to values close enough to unity for tidal circularization to transform the inner planet to a close-in planet. Since a moderate eccentricity can remain for the close-in planet, this mechanism may account for the observed close-in planets with moderate eccentricities and without nearby secondary planets. Since these planets also remain a broad range of orbital inclinations (even retrograde ones), the contribution of this process would be clarified by more observations of Rossiter-McLaughlin effects for transiting planets.