Eccentricity oscillation of the planet orbiting 16 Cyg B

M. Holman (CITA/U. Toronto), J. Touma (U. Texas, McDonald Obs.), S. Tremaine (CITA/U. Toronto)

The orbit of the recently discovered planet 16 Cyg Bb has semimajor axis a=1.72 AU and eccentricity e=0.67 (Cochran et al. 1997), the largest eccentricity of any known planet. This requires explanation, since planets that form from a disk are expected to have nearly circular orbits. We suggest that the eccentric orbit of 16 Cyg Bb is due to the gravitational tidal field from the distant companion star 16 Cyg A. If 16 Cyg Bb formed in a nearly circular orbit with its orbital plane inclined between tex2html_wrap_inline12 and tex2html_wrap_inline14 from the orbital plane of the binary, the eccentricity of its orbit would oscillate between low and high eccentricity with a period of tex2html_wrap_inline16 - tex2html_wrap_inline18 yr. The planet could spend up to tex2html_wrap_inline20 of its life with an eccentricity greater than 0.6. Our hypothesis requires that there is less than 1 Jupiter mass in other material within 30 AU of 16 Cyg B, and is consistent with the low eccentricities of the planets in other binary systems. Our results imply that the secular evolution of planetary orbits in many binary systems commonly leads to high eccentricities and dynamical chaos, and occasionally to collisions with the primary star.