On the Origin of Massive Eccentric Planets

We propose a merger scenario for the newly discovered extrasolar planets around 70 Vir (Marcy Marcy & Butler). These planets have mass Mp sin i = 6.6 and 9MJ (where MJ is Jupiter's mass and i is the orbital inclination), orbital semimajor axis a = 0.43 and 0.34 AU, and eccentricity e = 0.38 and 0.35, respectively. Our scenario is based on the conventional formation model of giant planets (gas accretion onto solid cores) and the long-term orbital stability theory of planetary systems. We suggest that in a relatively massive disk, several giant planets can be formed with Mp ~ 1-3MJ and a ≳ 1 AU. Under the persistence of the disk gas, the protogiant planet system is stable during its formation epoch (within 106-107 yr). But, after the depletion of the disk gas, mutual gravitational perturbation between the planets induces a gradual increase in their orbital eccentricities, until their orbits become unstable and begin to cross each other. We present numerical calculations of the orbital evolution leading to the orbit crossing stage. Our results indicate that the inner planets have a tendency to merge into a massive planet with relatively high e (≃0.2-0.9) and small a (≃0.5-1 AU). The orbital decay is a result of the gravitational perturbation by the outer planets and the dissipation of the colliding planets' relative kinetic energy. Afterward, long-term perturbation would slightly reduce the merged body's a, while it would keep its e high. The orbital properties of the merged body are consistent with those of the massive eccentric planets around 70 Vir and HD 114762. The onset timescale for orbit crossing within a planetary system is sensitively determined by the planets' mass and separation, which may explain the diversity in the orbital properties among the newly discovered planetary systems.