Protoplanetary formation. I. Neptune

Neptune has a gaseous envelope with mass larger than that of the Earth. We examine the possibility that proto-Neptune formed through an initial buildu p of a core prior to the accretion of a gaseous envelope in the solar nebul a environment. In thin conventional scenario, the inferred formation timesc ale of proto-Neptune would be similar to 10(6-7) yr since the signature of protoplanetary disks are observed to vanish on such a timescale. We show th at after their emergence through an initial epoch of runaway buildup, proto -Neptune's core would exert gravitational perturbations On and excite eccen tricity among the planetesimals near its orbit. In the absence of dissipati on due to inelastic collisions or gas drag, the planetesimals' velocity dis persion and the core's growth timescale would increase indefinitely. But cl ose encounters between planetesimals and the core also lead to orbital drif t and migration, which would replenish the planetesimals in the core's feed ing zone and thereby sustain the core's rate of growth. In a subsequent pap er, we show that when the core has acquired an adequate (comparable to that of Neptune's present) mass, it may capture those planetesimals along its m igration path onto its strongest mean-motion resonances. Such a process wou ld account for the observed distribution of trans-Neptunian objects. Eventu ally, this resonant barrier would prevent residual planetesimals from reach ing the core's feeding zone and terminate its mass growth as well as its or bital migration. We also investigate the process of gas accretion onto the core. We show that the depletion of nebula gas may be required to limit the mass of proto-Neptune's gaseous envelope. Such a depletion may either be d ue to the global viscous evolution of the nebula or to tidally induced gay, formation near proto-Neptune's orbit. The former process would require a 3 order of magnitude seduction of the surface density from that of the minim um-mass solar nebula model. The latter process would provide not only a nat ural mass limit for the gaseous envelope in both Neptune and Uranus but als o a mechanism for Neptune's outward orbital migration on its formation time scale. The necessary conditions for gap formation are that the outer parts of the disk have a very low viscosity and a small thickness.