Protoplanetary Formation. I. Neptune

Abstract

Neptune has a gaseous envelope with mass larger than that of the Earth. We examine the possibility that proto-Neptune formed through an initial buildup of a core prior to the accretion of a gaseous envelope in the solar nebula environment. In this conventional scenario, the inferred formation timescale of proto-Neptune would be ~106-7 yr since the signature of protoplanetary disks are observed to vanish on such a timescale. We show that after their emergence through an initial epoch of runaway buildup, proto-Neptune's core would exert gravitational perturbations on and excite eccentricity among the planetesimals near its orbit. In the absence of dissipation due to inelastic collisions or gas drag, the planetesimals' velocity dispersion and the core's growth timescale would increase indefinitely. But close encounters between planetesimals and the core also lead to orbital drift and migration, which would replenish the planetesimals in the core's feeding zone and thereby sustain the core's rate of growth. In a subsequent paper, 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 migration path onto its strongest mean-motion resonances. Such a process would account for the observed distribution of trans-Neptunian objects. Eventually, this resonant barrier would prevent residual planetesimals from reaching the core's feeding zone and terminate its mass growth as well as its orbital 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 due to the global viscous evolution of the nebula or to tidally induced gap formation near proto-Neptune's orbit. The former process would require a 3 order of magnitude reduction of the surface density from that of the minimum-mass solar nebula model. The latter process would provide not only a natural mass limit for the gaseous envelope in both Neptune and Uranus but also a mechanism for Neptune's outward orbital migration on its formation timescale. The necessary conditions for gap formation are that the outer parts of the disk have a very low viscosity and a small thickness.