Evolution of circumbinary planets around eccentric binaries: The case of Kepler-34

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
e-mail: wilhelm.kley@uni-tuebingen.de
² Institute for Astronomy, University of Hawaii-Manoa, Honolulu, HI 96825, USA
e-mail: nader@ifa.hawaii.edu
³ Institut für Theoretische Astrophysik, 69120 Heidelberg, Germany

Received: 1 June 2015
Accepted: 22 June 2015

Abstract

The existence of planets orbiting a central binary star system immediately raises questions regarding their formation and dynamical evolution. Recent discoveries of circumbinary planets by the Kepler space telescope has shown that some of these planets reside close to the dynamical stability limit where the strong perturbations induced by the binary makes it very difficult to form planets in situ. For binary systems with nearly circular orbits, such as Kepler-38, the observed proximity of planetary orbits to the stability limit can be understood by an evolutionary process in which planets form farther out in the calmer environment of the disk and migrate in toward their observed position. The Kepler-34 system is different from other systems because it has a high orbital eccentricity of 0.52. We analyzed evolutionary scenarios for the planet observed around this system using two-dimensional hydrodynamical simulations. The highly eccentric binary opens a wide inner hole in the disk that is also eccentric and displays a slow prograde precession. As a result of the large eccentric inner gap, an embedded planet settles in a final equilibrium position that lies beyond the observed location of Kepler-34 b, but has the correct eccentricity. In this configuration, the planetary orbit is aligned with the disk in a state of apsidal corotation. To account for the closer orbit of Kepler-34 b to the central binary, we considered a two-planet scenario and examined the evolution of the system through joint inward migration and capture into mean-motion resonances. When the inner planet orbits inside the gap of the disk, planet-planet scattering ensues. While often one object is thrown into a large, highly eccentric orbit, the system is at times left with a planet close to the observed orbit, suggesting that Kepler-34 might have had two circumbinary planets where one might have been scattered out of the system or into an orbit where it did not transit the central binary during the operation of Kepler.