  This paper constructs an analytic description for the late stages of giant
planet formation. During this phase of evolution, the planet gains the majority
of its final mass through gas accretion at a rapid rate. This work determines
the density and velocity fields for material falling onto the central planet
and its circumplanetary disk, and finds the corresponding column density of
this infalling envelope. We derive a steady-state solution for the surface
density of the disk as a function of its viscosity (including the limiting case
where no disk accretion occurs). Planetary magnetic fields truncate the inner
edge of the disk and determine the boundary conditions for mass accretion onto
the planet from both direct infall and from the disk. The properties of the
forming planet and its circumplanetary disk are determined, including the
luminosity contributions from infall onto the planet and disk surfaces, and
from disk viscosity. The radiative signature of the planet formation process is
explored using a quasi-spherical treatment of the emergent spectral energy
distributions. The analytic solutions developed herein show how the protoplanet
properties (envelope density distribution, velocity field, column density, disk
surface density, luminosity, and radiative signatures) vary with input
parameters (instantaneous mass, orbital location, accretion rate, and planetary
magnetic field strength).
