ON THE ORIGIN OF MASSIVE ECCENTRIC PLANETS

We propose a merger scenario for the newly discovered extrasolar plane ts around 70 Vir (Marcy & Butler) and HD 114762 (Latham, Stefanik, & M azeh; Marcy & Butler). These planets have mass M(p) sin i=6.6 and 9M(J ) (where M(J) is Jupiter's mass and i is the orbital inclination), orb ital 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-t erm orbital stability theory of planetary systems. We suggest that in a relatively massive disk, several giant planets can be formed with M( p) similar to 1-3M(J) and a greater than or similar to 1 AU. Under the persistence of the disk gas, the protogiant planet system is stable d uring its formation epoch (within 10(6)-10(7) yr). But, after the depl etion of the disk gas, mutual gravitational perturbation between the p lanets induces a gradual increase in their orbital eccentricities, unt il their orbits become unstable and begin to cross each other. We pres ent numerical calculations of the orbital evolution leading to the orb it crossing stage. Our results indicate that the inner planets have a tendency to merge into a massive planet with relatively high e (simila r or equal to 0.2-0.9) and small a (similar to 0.5-1 AU). The orbital decay is a result of the gravitational perturbation by the outer plane ts and the dissipation of the colliding planets' relative kinetic ener gy. Afterward, long-term perturbation would slightly reduce the merged body's a, while it would keep its e high. The orbital properties of t he merged body are consistent with those of the massive eccentric plan ets around 70 Vir and HD 114762. The onset timescale for orbit crossin g within a planetary system is sensitively determined by the planets' mass and separation, which may explain the diversity in the orbital pr operties among the newly discovered planetary systems.