First-versus second-generation planet formation in post-common envelope binary (PCEB) planetary systems

Abstract

We examine planets orbiting post-common envelope binaries from the perspective of angular momentum evolution, and conclude that the planets are more likely to be first-generation (FG) planets than second generation (SG) planets. FG planets were born together with the parent stars, while SG planets form later from an SG protoplanetary disc formed by mass-loss from the evolved primary star during its red giant branch phase or asymptotic giant branch phase. We find that in some systems the SG scenario requires that more than 20 per cent of the SG protoplanetary disc mass ends in planets. Although we cannot rule out SG planet formation in these systems, this fraction of mass that ends in planets is much higher than the value commonly used in planet formation theories. On the other hand, we find that for each of the systems, we can build a progenitor system composed of a main-sequence binary system orbited by the appropriate planets. This can be done if the secondary star was in a resonance with the inner planet. To account for the progenitor properties, we suggest that in cases where the secondary star has a mass of ~0.1-0.2M⊙, it was formed in the same way planets are formed, i.e. from a disc.