Direct imaging (DI) surveys suggest that gas giants beyond 20 au are rare around FGK stars. However, it is not clear what this means for the formation frequency of gravitational instability (GI) protoplanets due to uncertainties in gap opening and migration efficiency. Here we combine state-of-the-art calculations of homogeneous planet contraction with a population synthesis code. We find DI constraints to be satisfied if protoplanet formation by GI occurs in tens of per cent of systems if protoplanets 'supermigrate' to small separations. In contrast, GI may occur in only a few per cent of systems if protoplanets remain stranded at wide orbits because their migration is 'quenched' by efficient gap opening. We then use the frequency of massive giants in radial velocity surveys inside 5 au to break this degeneracy - observations recently showed that this population does not correlate with the host star metallicity and is therefore suspected to have formed via GI followed by inward migration. We find that only the supermigration scenario can sufficiently explain this population while simultaneously satisfying the DI constraints and producing the right mass spectrum of planets inside 5 au. If massive gas giants inside 5 au formed via GI, then our models imply that migration must be efficient and that the formation of GI protoplanets occurs in at least tens of per cent of systems.
Bibliographical noteFunding Information:
JH and SN acknowledge support from the Science and Technology Funding Council (STFC) grants ST/N504117/1 and ST/N000757/1, as well as the STFC DiRAC HPC Facility (grant ST/H00856X/1 and ST/K000373/1). DiRAC is part of the National E-Infrastructure. MB gratefully acknowledges support from STFC grant ST/M001229/1 and RH acknowledges support from SNSF grant 200021 169054. Part of this work was conducted within the framework of the National Centre for Competence in Research PlanetS, supported by the Swiss National Foundation. We would also like to acknowledge useful comments from the anonymous referee that helped to clarify some issues in this paper.
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
- Accretion discs
- Brown dwarfs
- Planet-disc interactions
- Planets and satellites: composition
- Planets and satellites: formation
- Protoplanetary discs