A desert of gas giant planets beyond tens of au

Abstract

Direct imaging observations constrain the fraction of stars orbited by gas giant planets with separations greater than 10 au to about 0.01 only. This is widely believed to indicate that massive protoplanetary discs rarely fragment on planetary mass objects. I use numerical simulations of gas clumps embedded in massive gas discs to show that these observations are consistent with $\sim 0.2 - 10$ planetary mass clumps per star being born in young gravitationally unstable discs. A trio of processes -- rapid clump migration, tidal disruption and runaway gas accretion -- destroys or transforms all of the simulated clumps into other objects, resulting in a desert of gas giants beyond separation of approximately 10 au. The cooling rate of the disc controls which of the three processes is dominant. For cooling rates faster than a few local dynamical times, clumps always grow rapidly and become massive brown dwarfs or low mass stars. For longer cooling times, post-collapse (high density) planets migrate inward to $\sim 10-20$ au where they open a gap in the disc and then continue to migrate inward much less rapidly. Pre-collapse (low density) planets are tidally disrupted and may leave massive solid cores behind. Gas giant planets observed inside the desert, such as those in HR 8799, must have followed an unusual evolutionary path, e.g., their host disc being dispersed in a catastrophic fashion.