Chaotic variations in the eccentricity of the planet orbiting 16 Cygni B

Abstract

The planet recently discovered¹ orbiting the star 16 Cyg B has the largest eccentricity (e= 0.67) of any known planet. Planets that form in circumstellar disks are expected to have nearly circular orbits, although gravitational interactions in a system of two or more planets could generate high-eccentricity orbits^2,3. Here we suggest that the eccentric orbit of 16 Cyg Bb arises from gravitational interactions with the distant companion star, 16 Cyg A. Assuming that 16 Cyg Bb formed in a nearly circular orbit, with the orbital plane inclined between 45° and 135° to the orbital plane of 16 Cyg A, and that there are no other planets with a mass similar to that of Jupiter within 30 astronomical units (AU, the average distance between the Earth and the Sun), then 16 Cyg Bb will oscillate between low-eccentricity and high-eccentricity orbits. The transitions between these orbits should occur every 10⁷–10⁹ years, with the planet spending up to 35 per cent of its lifetime with an eccentricity e> 0.6. These results imply that planetary orbits in binary stellar systems commonly experience periods of high eccentricity and dynamical chaos, and that such planets may occasionally collide with the primary star.