Outward Migration of Terrestrial Embryos in Binary Systems

TL;DR

This study uses n-body simulations to show that terrestrial embryos in tight binary systems can migrate outward beyond initial formation zones, potentially forming habitable planets despite initial growth suppression.

Contribution

It demonstrates that embryo migration in binary systems can extend beyond critical formation regions, challenging previous assumptions about planet formation limits.

Findings

Embryos can migrate out to 0.9-1.2 AU within 10^6-10^7 years.

Migration extent increases with higher solid surface density and flatter profiles.

Outer-most embryo migration is largely independent of the critical radius a_{crit}.

Abstract

We consider the formation and migration of protoplanetary embryos in disks around the stars in tight binary systems (separations ~ 20 AU. In such systems, the initial stages of runaway embryo formation are expected to only take place within some critical disk radius a_{crit}, due to the perturbing effect of the binary companions (Thebault et al. 2009). We perform n-body simulations of the evolution of such a population of inner-disk embryos surrounded by an outer-disk of smaller planetesimals. Taking Alpha Centauri-B as our fiducial reference example in which a_{crit} ~ 0.7 AU, and using a Minimum Mass Nebular Model with $\Sigma \propto a^{-3/2}$, we find that within 10^6 yrs (10^7 yrs), systems will on average contain embryos which have migrated out to 0.9 AU (1.2 AU), with the average outer-most body having a mass of 0.2 M_{earth} 0.4 M_{earth}. Changes to increase the surface density of solids or to use a flatter profile both produce increased embryo migration and growth. At a given time, the relative change in semi-major axis of the outer-most embryo in these simulations is found to be essentially independent of a_{crit}, and we note that little further embryo migration takes place beyond 10^7 years. We conclude that the suppression of runaway growth outside a_{crit} does not mean that the habitable zones in such tight binary systems will be devoid of detectable, terrestrial mass planets, even if a_{crit} lies significantly interior to the inner edge of the habitable zone.