Binary Planet Formation by Gas-assisted Encounters of Planetary Embryos

Abstract

Abstract We present radiation hydrodynamic simulations in which binary planets form by close encounters in a system of several super-Earth embryos. The embryos are embedded in a protoplanetary disk consisting of gas and pebbles and evolve in a region where the disk structure supports convergent migration due to Type I torques. As the embryos accrete pebbles, they become heated and thus affected by the thermal torque and the hot-trail effect, which excites orbital eccentricities. Motivated by findings of Eklund & Masset, we assume that the hot-trail effect also operates vertically and reduces the efficiency of inclination damping. Non-zero inclinations allow the embryos to become closely packed and also vertically stirred within the convergence zone. Subsequently, close encounters of two embryos assisted by the disk gravity can form transient binary planets that quickly dissolve. Binary planets with a longer lifetime of ∼104 yr form in three-body interactions of a transient pair with one of the remaining embryos. The separation of binary components generally decreases in subsequent encounters and because of pebble accretion until the binary merges, forming a giant planet core. We provide an order-of-magnitude estimate of the expected occurrence rate of binary planets, yielding one binary planet per ≃(2–5) × 104 planetary systems. Therefore, although rare, binary planets may exist in exoplanetary systems and they should be systematically searched for.