  Gas giant planets may form early-on during the evolution of protostellar
discs, while these are relatively massive. We study how Jupiter-mass
planet-seeds (termed protoplanets) evolve in massive, but gravitationally
stable (Q>1.5), discs using radiative hydrodynamic simulations. We find that
the protoplanet initially migrates inwards rapidly, until it opens up a gap in
the disc. Thereafter, it either continues to migrate inwards on a much longer
timescale or starts migrating outwards. Outward migration occurs when the
protoplanet resides within a gap with gravitationally unstable edges, as a high
fraction of the accreted gas is high angular momentum gas from outside the
protoplanet's orbit. The effect of radiative heating from the protoplanet is
critical in determining the direction of the migration and the eccentricity of
the protoplanet. Gap opening is facilitated by efficient cooling that may not
be captured by the commonly used beta-cooling approximation. The protoplanet
initially accretes at a high rate (1e-3Mj/yr), and its accretion luminosity
could be a few tenths of the host star's luminosity, making the protoplanet
easily observable (albeit only for a short time). Due to the high gas accretion
rate, the protoplanet generally grows above the deuterium-burning mass-limit.
Protoplanet radiative feedback reduces its mass growth so that its final mass
is near the brown dwarf-planet boundary. The fate of a young planet-seed is
diverse and could vary from a gas giant planet on a circular orbit at a few AU
from the central star to a brown dwarf on an eccentric, wide orbit.
