2014MNRAS.442.2296P

Planets close to their stars are thought to form farther out and migrate inwards due to angular momentum exchange with gaseous protoplanetary discs. This process can produce systems of planets in co-orbital (Trojan or 1:1) resonance, in which two planets share the same orbit, usually separated by 60 deg. Co-orbital systems are detectable among the planetary systems found by the Kepler mission either directly or by transit timing variations. However, no co-orbital systems have been found within the thousands of Kepler planets and candidates. Here, we study the orbital evolution of co-orbital planets embedded in a protoplanetary disc using a grid-based hydrodynamics code. We show that pairs of similar-mass planets in co-orbital resonance are disrupted during large-scale orbital migration. Destabilization occurs when one or both planets are near the critical mass needed to open a gap in the gaseous disc. A confined gap is opened that spans the 60 deg azimuthal separation between planets. This alters the torques imparted by the disc on each planet - pushing the leading planet outwards and the trailing planet inwards - and disrupts the resonance. The mechanism applies to systems in which the two planets' masses differ by a factor of 2 or less. In a simple flared disc model, the critical mass for gap opening varies from a few Earth masses at the inner edge of the disc to 1 Saturn mass at 5 au. A pair of co-orbital planets with masses in this range that migrates will enter a region where the planets are at the gap-opening limit. At that point, the resonance is disrupted. We therefore predict an absence of planets on co-orbital configurations with masses in the super-Earth to Saturn mass range with similar masses.