28.05
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 
and
from the orbital plane of the binary, the eccentricity
of its orbit would oscillate between low and high eccentricity with a
period of 
- 
yr. The planet could spend up to 
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.