Planet formation via core accretion
Collaborator - Phil Armitage (JILA/University of
The most commonly accepted mechanism for the formation of Jupiter-like planets is the core
accretion model. In this model a rocky core forms through the coagulation of planetesimals
until it is sufficiently massive to accrete a gaseous envelope. Initially this envelope is in
hydrostatic equilibrium, with most of the luminosity provided by the accreting planetesimals. Once
the core reaches a critical mass, however, hydrostatic equilibrium is no longer possible, and a
phase of rapid gas accretion occurs.
There are, however, a number of issues with this model.
The dust grains from which the planetesimals form may undergo a phase of rapid inward
migration when they reach a certain size, the core itself, once it reaches
about an Earth mass, should also migrate inwards rapidly, and upper limits for
Jupiter's core are smaller than most theoretical estimates. Detailed models also
predict growth timescales that may easily exceed the lifetime of the gaseous disc from
which the planet formed.
Recent work on self-gravitating discs has already suggested
that the grain growth may be accelerated by concentrating the grains in the center of the
self-gravitating structures, possibly overcoming the issue of the rapid grain migration.
Other simulations of Earth mass-like cores in turbulent discs, by Richard Nelson and John
Papaloizou at Queen Mary University in London, have also suggested that the core may
undergo a random walk, rather than migrating in a single direction. What we find, when
including this random walk in the standard core accretion model, is that the core growth
accelerates and reaches a critical mass far more quickly than if the core is assumed to
remain stationary. What is more, when the planetesimal density is reduced slightly, we find
that the critical core mass is consistent with Jupiter's measured core mass.
On the formation time scale and core masses of gas giant planets, W.K.M. Rice and P.J. Armitage, ApJL, 598, L55 (2003)