EVIDENCE FROM THE ASTEROID BELT FOR A VIOLENT PAST EVOLUTION OF JUPITER'S ORBIT

Abstract

We use the current orbital structure of large (>50km) asteroids in the main\nasteroid belt to constrain the evolution of the giant planets when they\nmigrated from their primordial orbits to their current ones. Minton & Malhotra\n(2009) showed that the orbital distribution of large asteroids in the main belt\ncan be reproduced by an exponentially-decaying migration of the giant planets\non a time scale of tau ~ 0.5My. However, self-consistent numerical simulations\nshow that the planetesimal-driven migration of the giant planets is\ninconsistent with an exponential change in their semi major axes on such a\nshort time scale (Hahn & Malhotra, 1999). In fact, the typical time scale is\ntau > 5My. When giant planet migration on this time scale is applied to the\nasteroid belt, the resulting orbital distribution is incompatible with the\nobserved one. However, the planet migration can be significantly sped up by\nplanet-planet encounters. Consider an evolution where both Jupiter and Saturn\nhave close encounters with a Neptune-mass planet (presumably Uranus or Neptune\nthemselves) and where this third planet, after being scattered inwards by\nSaturn, is scattered outwards by Jupiter. This scenario leads to a very rapid\nincrease in the orbital separation between Jupiter and Saturn that we show here\nto have only mild effects on the structure of asteroid belt. This type of\nevolution is called a jumping-Jupiter case. Our results suggest that the total\nmass and dynamical excitation of the asteroid belt before migration were\ncomparable to those currently observed. Moreover, they imply that, before\nmigration, the orbits of Jupiter and Saturn were much less eccentric than the\ncurrent ones.\n