Dust traps as planetary birthsites: basics and vortex formation

We present a simple model for low-mass planet formation and subsequent evolution within "transition" discs. We demonstrate quantitatively that the predicted and observed structure of such discs are prime birthsites of planets. Planet formation is likely to proceed through pebble accretion, should a planetary embryo ($M\gtrsim 10^{-4}\,$M$_\oplus$) form. Efficient pebble accretion is likely to be unavoidable in transition disc dust traps, as the size of the dust particles required for pebble accretion are those which are most efficiently trapped in the transition disc dust trap. Rapid pebble accretion within the dust trap gives rise, not only to low-mass planets, but to a large accretion luminosity. This accretion luminosity is sufficient to heat the disc outside the gravitational influence of the planet and makes the disc locally baroclinic, and a source of vorticity. Using numerical simulations we demonstrate that this source of vorticity can lead to the growth of a single large scale vortex in $\sim 100$ orbits, which is capable of trapping particles. Finally, we suggest an evolutionary cycle: planet formation proceeds through pebble accretion, followed by vortex formation and particle trapping in the vortex quenching the planetary accretion and thus removing the vorticity source. After the vortex is destroyed the process can begin anew. This means transition discs should present with large scale vortices for a significant fraction of their lifetimes and remnant planets at large ~10AU separation should be a common outcome of this cycle.