The diverse lives of massive protoplanets in self-gravitating discs

TL;DR

This study uses radiative hydrodynamic simulations to explore the evolution of Jupiter-mass protoplanets in massive, gravitationally stable discs, revealing diverse migration paths, accretion behaviors, and final masses near the brown dwarf-planet boundary.

Contribution

It provides new insights into how radiative heating and cooling influence protoplanet migration, accretion, and final mass in self-gravitating discs, highlighting diverse evolutionary outcomes.

Findings

Protoplanets initially migrate inward rapidly before gap formation.

Outward migration occurs within unstable gap edges due to high angular momentum gas.

Radiative feedback influences protoplanet mass growth and migration direction.

Abstract

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.