Planet Migration and Binary Companions: The Case of HD 80606b

Abstract

The exo-solar planet HD 80606b has a highly eccentric (e=0.93) and tight (a=0.47 AU) orbit. We study how it might arrive at such an orbit and how it has avoided being tidally circularized until now. The presence of a stellar companion to the host star suggests the possibility that the Kozai mechanism and tidal dissipation combined to draw the planet inward well after it formed: Kozai oscillations produce periods of extreme eccentricity in the planet orbit, and the tidal dissipation that occurs during these periods of small pericentre distances leads to gradual orbital decay. We call this migration mechanism the 'Kozai migration'. It requires that the initial planet orbit is highly inclined relative to the binary orbit. For a companion at 1000 AU and an initial planet orbit at 5 AU, the minimum relative inclination required is ~ 85. We discuss the efficiency of tidal dissipation inferred from the observations of exo-planets. Moreover, we investigate possible explanations for the velocity residual (after the motion induced by the planet is removed) observed on the host star: a second planet in the system is excluded over a large extent of semi-major axis space if Kozai migration is to work, and the tide raised on the star by HD 80606b is likely too small in amplitude. Lastly, we discuss the relevance of Kozai migration for other planetary systems.