Formation of a wide-orbit giant planet in a gravitationally unstable subsolar-metallicity protoplanetary disc

Abstract

Direct imaging observations of planets revealed that wide-orbit ($>10$ au) giant planets exist even around subsolar-metallicity host stars and do not require metal-rich environments for their formation. A possible formation mechanism of wide-orbit giant planets in subsolar-metallicity environments is the gravitational fragmentation of massive protoplanetary discs. Here, we follow the long-term evolution of the disc for 1 Myr after its formation, which is comparable to disc lifetime, by way of a two-dimensional thin-disc hydrodynamic simulation with the metallicity of 0.1 ${\rm Z}_{\odot}$. We find a giant protoplanet that survives until the end of the simulation. The protoplanet is formed by the merger of two gaseous clumps at $\sim$0.5 Myr after disc formation, and then it orbits $\sim$200 au from the host star for $\sim$0.5 Myr. The protoplanet's mass is $\sim$10 ${\rm M}_{\rm J}$ at birth and gradually decreases to 1 ${\rm M}_{\rm J}$ due to the tidal effect from the host star. The result provides the minimum mass of 1 ${\rm M}_{\rm J}$ for protoplanets formed by gravitational instability in a subsolar-metallicity disc. We anticipate that the mass of a protoplanet experiencing reduced mass loss thanks to the protoplanetary contraction in higher resolution simulations can increase to $\sim$10 ${\rm M}_{\rm J}$. We argue that the disc gravitational fragmentation would be a promising pathway to form wide-orbit giant planets with masses of $\ge1$ ${\rm M}_{\rm J}$ in subsolar-metallicity environments.