Influence of an inner disc on the orbital evolution of massive planets migrating in resonance

Abstract

\n Context. The formation of resonant planet pairs in exoplanetary systems\ninvolves planetary migration inside the protoplanetary disc. After a\nresonant capture, subsequent migration leads to a large increase in\nplanetary eccentricities, if no damping mechanism is applied. This has\nled to the conclusion that the migration of resonant planetary systems\ncannot take place across large radial distances, but must be\nterminated rapidly by disc dissipation.\n Aims. We investigate if the presence of an inner disc could supply\neccentricity damping to the inner planet, and if this effect could\nexplain observed eccentricities in some planetary systems.\n Methods. We compute hydrodynamic simulations of giant planets, in\norbits of a given eccentricity about an inner gas disc, and measure\nthe effect of the gas disc on the planetary orbital parameters. We\nperform detailed long-term calculations of the GJ 876 system. We\ncomplete N-body simulations, which include artificial forces on the\nplanets that recreate the effect of the inner and outer discs.\n Results. We find that we cannot neglect the influence of the inner disc,\nand that the disc could explain the observed eccentricities. In\nparticular, we reproduce the orbital parameters of a few\nsystems engaged in 2:1 mean motion resonances: GJ 876, HD 73 526,\nHD 82 943, and HD 128 311. Analytically, we derive the effect\nthat the inner disc should have on the inner planet to reach a\nspecific orbital configuration, for any given damping effect of the\nouter disc on the outer planet.\n Conclusions. We conclude that an inner disc, even though difficult to model\nproperly in hydrodynamical simulations, should be taken into account\nbecause of its damping effect on the eccentricity of the inner planet.\nBy including this effect, we can explain quite naturally the observed\norbital elements of the pairs of known resonant exoplanets.\n